Implement fvar InstanceRecord parsing

[Kumasan00]

Body:

This PR adds support for parsing Instance Records from the fvar table, as proposed in #208 .

Changes:

• Added Instance struct to represent a single instance record.
• Added Instances struct and InstancesIter for iterating over records.
• Updated fvar::Table to include a public instances field.
• Implemented logic to detect the presence of the optional postScriptNameID field based on record_len and axisCount, strictly following the OpenType specification.

Example: Users can now iterate over named instances like this:

if let Some(fvar) = face.tables().fvar {
    for instance in fvar.instances.iter() {
        let name_id = instance.subfamily_name_id;
        let coords = instance.coordinates;
        // ...
    }
}

Related Issue: Closes #208 Relates to #129

[Kumasan00]

Verification: I verified the implementation by comparing the parsing results against the read-fonts crate using STIXTwoText[wght].ttf and NotoSans-VariableFont_wdth,wght.ttf. The outputs match perfectly.

Click to see the comparison output

STIXTwoText[wght].ttf :

ttf-parser output:

Instance 0: name_id=257, flags=0, coordinates=[Fixed(400.0)], postscript_name_id=None
Instance 1: name_id=258, flags=0, coordinates=[Fixed(500.0)], postscript_name_id=None
Instance 2: name_id=259, flags=0, coordinates=[Fixed(600.0)], postscript_name_id=None
Instance 3: name_id=260, flags=0, coordinates=[Fixed(700.0)], postscript_name_id=None
ttf-parser iterator output:
Instance via iterator: Instance { subfamily_name_id: 257, flags: 0, coordinates: [Fixed(400.0)], post_script_name_id: None }
Instance via iterator: Instance { subfamily_name_id: 258, flags: 0, coordinates: [Fixed(500.0)], post_script_name_id: None }
Instance via iterator: Instance { subfamily_name_id: 259, flags: 0, coordinates: [Fixed(600.0)], post_script_name_id: None }
Instance via iterator: Instance { subfamily_name_id: 260, flags: 0, coordinates: [Fixed(700.0)], post_script_name_id: None }
read-fonts output:
ReadFonts Instance 0: Ok(InstanceRecord { subfamily_name_id: NameId 257, flags: 0, coordinates: [400.0], post_script_name_id: None })
ReadFonts Instance 1: Ok(InstanceRecord { subfamily_name_id: NameId 258, flags: 0, coordinates: [500.0], post_script_name_id: None })
ReadFonts Instance 2: Ok(InstanceRecord { subfamily_name_id: NameId 259, flags: 0, coordinates: [600.0], post_script_name_id: None })
ReadFonts Instance 3: Ok(InstanceRecord { subfamily_name_id: NameId 260, flags: 0, coordinates: [700.0], post_script_name_id: None })

NotoSans-VariableFont_wdth,wght.ttf:

ttf-parser output:
Instance 0: name_id=297, flags=0, coordinates=[Fixed(100.0), Fixed(100.0)], postscript_name_id=Some(298)
Instance 1: name_id=299, flags=0, coordinates=[Fixed(200.0), Fixed(100.0)], postscript_name_id=Some(300)
Instance 2: name_id=301, flags=0, coordinates=[Fixed(300.0), Fixed(100.0)], postscript_name_id=Some(302)
Instance 3: name_id=303, flags=0, coordinates=[Fixed(400.0), Fixed(100.0)], postscript_name_id=Some(304)
Instance 4: name_id=305, flags=0, coordinates=[Fixed(500.0), Fixed(100.0)], postscript_name_id=Some(306)
Instance 5: name_id=307, flags=0, coordinates=[Fixed(600.0), Fixed(100.0)], postscript_name_id=Some(308)
Instance 6: name_id=309, flags=0, coordinates=[Fixed(700.0), Fixed(100.0)], postscript_name_id=Some(310)
Instance 7: name_id=311, flags=0, coordinates=[Fixed(800.0), Fixed(100.0)], postscript_name_id=Some(312)
Instance 8: name_id=313, flags=0, coordinates=[Fixed(900.0), Fixed(100.0)], postscript_name_id=Some(314)
ttf-parser iterator output:
Instance via iterator: Instance { subfamily_name_id: 297, flags: 0, coordinates: [Fixed(100.0), Fixed(100.0)], post_script_name_id: Some(298) }
Instance via iterator: Instance { subfamily_name_id: 299, flags: 0, coordinates: [Fixed(200.0), Fixed(100.0)], post_script_name_id: Some(300) }
Instance via iterator: Instance { subfamily_name_id: 301, flags: 0, coordinates: [Fixed(300.0), Fixed(100.0)], post_script_name_id: Some(302) }
Instance via iterator: Instance { subfamily_name_id: 303, flags: 0, coordinates: [Fixed(400.0), Fixed(100.0)], post_script_name_id: Some(304) }
Instance via iterator: Instance { subfamily_name_id: 305, flags: 0, coordinates: [Fixed(500.0), Fixed(100.0)], post_script_name_id: Some(306) }
Instance via iterator: Instance { subfamily_name_id: 307, flags: 0, coordinates: [Fixed(600.0), Fixed(100.0)], post_script_name_id: Some(308) }
Instance via iterator: Instance { subfamily_name_id: 309, flags: 0, coordinates: [Fixed(700.0), Fixed(100.0)], post_script_name_id: Some(310) }
Instance via iterator: Instance { subfamily_name_id: 311, flags: 0, coordinates: [Fixed(800.0), Fixed(100.0)], post_script_name_id: Some(312) }
Instance via iterator: Instance { subfamily_name_id: 313, flags: 0, coordinates: [Fixed(900.0), Fixed(100.0)], post_script_name_id: Some(314) }
read-fonts output:
ReadFonts Instance 0: Ok(InstanceRecord { subfamily_name_id: NameId 297, flags: 0, coordinates: [100.0, 100.0], post_script_name_id: Some(NameId 298) })
ReadFonts Instance 1: Ok(InstanceRecord { subfamily_name_id: NameId 299, flags: 0, coordinates: [200.0, 100.0], post_script_name_id: Some(NameId 300) })
ReadFonts Instance 2: Ok(InstanceRecord { subfamily_name_id: NameId 301, flags: 0, coordinates: [300.0, 100.0], post_script_name_id: Some(NameId 302) })
ReadFonts Instance 3: Ok(InstanceRecord { subfamily_name_id: NameId 303, flags: 0, coordinates: [400.0, 100.0], post_script_name_id: Some(NameId 304) })
ReadFonts Instance 4: Ok(InstanceRecord { subfamily_name_id: NameId 305, flags: 0, coordinates: [500.0, 100.0], post_script_name_id: Some(NameId 306) })
ReadFonts Instance 5: Ok(InstanceRecord { subfamily_name_id: NameId 307, flags: 0, coordinates: [600.0, 100.0], post_script_name_id: Some(NameId 308) })
ReadFonts Instance 6: Ok(InstanceRecord { subfamily_name_id: NameId 309, flags: 0, coordinates: [700.0, 100.0], post_script_name_id: Some(NameId 310) })
ReadFonts Instance 7: Ok(InstanceRecord { subfamily_name_id: NameId 311, flags: 0, coordinates: [800.0, 100.0], post_script_name_id: Some(NameId 312) })
ReadFonts Instance 8: Ok(InstanceRecord { subfamily_name_id: NameId 313, flags: 0, coordinates: [900.0, 100.0], post_script_name_id: Some(NameId 314) })