Expand diagnostic excerpts using heuristics on syntactic information from TreeSitter (#21942)

This is quite experimental and untested in languages other than Rust.
It's written to attempt to do something sensible in many languages. Due
to its experimental nature, just releasing to staff, and so not
including it in release notes. Future release note might be "Improved
diagnostic excerpts by using syntactic info to determine the context
lines to show."

Release Notes:

- N/A