Add convenience methods to StringScanner

[jneen]

Depends on #16557.
Extracted from #16455.
Fixes #11259, with the exception of the methods that mutate the string.

Rationale

Ruby's comparable StringScanner library provides a number of extra convenience methods. I've gone through the list and implemented/specced the ones that make sense here.

Changes

• #matched? : Bool to check for the presence of @last_match
• #current_char : Char, #previous_char : Char as well as nilable variants
• #current_byte : UInt8, #previous_byte : UInt8, as well as nilable variants
• #beginning_of_line? I believe originally conceived because anchors like ^ were non-functional in Ruby due to a bug in their implementation - not an issue in Crystal. It is generally a useful method though, and I couldn't see an argument against including it.
• #rewind(Int) to move the scan head backwards manually
• #unscan for rewinding to the start of the previous scan Removed as this has some deep issues in its interactions with #check and #scan_until. Will open a separate PR for it later.

[jneen]

I think the spec failure here is a flapper, it is a TCP error unrelated to the change.

[jneen]

Updated docs.

Also realized there's a subtle footgun with #unscan, which is that if the last match was from a #check(...) call, it will still rewind.

[jneen]

And yeah having played around with it for a bit, I think this is a tricky enough issue to warrant removing #unscan from this PR and doing a separate one. I think it may require state alongside @last_match.