Add Haxe language support via vshaxe/haxe-language-server

Includes full setup with ls provider, tests, docs, etc.

test/conftest.py

@@ -255,6 +255,7 @@ _LANGUAGE_PYTEST_MARKERS: dict[Language, list[MarkDecorator | Mark]] = {
     Language.CSHARP: [pytest.mark.csharp],
     Language.FSHARP: [pytest.mark.fsharp],
     Language.GO: [pytest.mark.go],
+    Language.HAXE: [pytest.mark.haxe],
     Language.JAVA: [pytest.mark.java],
     Language.KOTLIN: [pytest.mark.kotlin, pytest.mark.skipif(is_ci, reason="Kotlin LSP JVM crashes on restart in CI")],
     Language.LEAN4: [pytest.mark.lean4, pytest.mark.skipif(_sh.which("lean") is None, reason="Lean is not installed")],