[display] Remove legacy diagnostics

[kLabz]

⚠️ Merge later

Should be merged sometime next year I suppose?

Closes #8086

[Simn]

Should we do this now?

[kLabz]

Well, hmm

• Latest vshaxe release still doesn't support Json RPC diagnostics (merged but not released)
• The number of people that have actually used Json RPC diagnostics is likely < 5 atm; we might want to release Haxe 5 preview with both, and then merge this

... which actually tells me there's potentially something missing before doing a vshaxe release: there's currently no way to opt out of Json RPC diagnostics, and in short term that might be annoying if it ends up being "broken" 🤔

cc @AlexHaxe

[AlexHaxe]

I think we should put a useLegacyDiagnostics config option in vshaxe. and it probably should default to false so that people use new diagnostics in 4.3.5+ and 5, because nobody's gonna turn it on unless it's heavily advertised.

and then we should release a new vshaxe version soonish. I have two fixes for tokentree that should go in.
there are a couple of issues with rename lib, but I haven't had the time to fix those. they can go into a later release when they are ready so it shouldn't be blocking a Json RPC enabled vshaxe release.

[kLabz]

Note that such a config option wouldn't work with Haxe 5 (and, more importantly, would make diagnostics fail if vshaxe tries to honor it on a Haxe version where this PR is merged)

[AlexHaxe]

true. we could use context.haxeServer.haxeVersion to limit it to supported versions (e.g. < 5.0.0) once this PR is merged. obviously that would leave nightlies without Json RPC with no diagnostics. but at least the stable release people will have an opt-out (not sure if there are people relying on nightlies for their projects).