Json rpc diagnostics

[kLabz]

• Use Json RPC diagnostics if available (added in [display] diagnostics as json rpc HaxeFoundation/haxe#11412)
• Removed (undocumented) populateCacheFromDisplay config, which is not needed with Json RPC diagnostics nor with recent nightlies; could technically be useful for older nightlies or 4.3.x but this hack has been removed so it's a bit of a PITA to continue to support it here
• Had to update tests because [js] Use native maps when ES6 is enabled. HaxeFoundation/haxe#11698 changed iteration order for js maps
• Added support for diagnostics on all open (Haxe) files in a single diagnostics request; can be disabled by config
  • TODO followup: add support for this config in vshaxe

[Simn]

I'm not very familiar with this code but at a glance it looks good.

[AlexHaxe]

I have only briefly run it on actual code, it seemed to work, but I haven't done any extensive testing.

code style wise curlies around single line bodies are preferred, but you'll easily find samples of rule breaking code throughout the project, so not a big deal (it might summon a Gama11).

other than that I see no blockers.

[kLabz]

I fixed the curlies and prepared HaxeFoundation/haxe#11707 for Json RPC diagnostics on 4.3.5.