fix(syntaxes): keyword token for track

[dannymcgee]

This PR updates the TM grammar for flow-control block expressions to tokenize track as a keyword.

Changes made

• Separate the valid sub-expressions of a @for/@if header into their own "clause" patterns (helps to avoid highlighting track as a keyword in the wrong context)
• Assign keyword.control.track.ng scope to the track token at the beginning of a "track-by" clause
• The item binding in item of items is now constant instead of readwrite, following the same logic mentioned in fix(syntaxes): variable token for @let binding #2160. Again, happy to revert this change if that's unwanted.

Screenshots

Before:

After:

[atscott]

Nice, thanks for this

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