fix: resolve .hx extension conflict with Haxe

Neovim's built-in filetype detection maps .hx to 'haxe', which
caused the test framework to attempt loading haxe.so instead of
helixql.so.

Changes:
- Renamed test file from test.hx to test.helixql
- Added vim modeline to ensure correct filetype detection
- Removed helixql filetype mapping (not needed and doesn't override
  Neovim's built-in detection anyway)

tests/query/highlights/helixql/test.helixql

@@ -120,3 +120,5 @@ QUERY testLiterals() =>
   b <- true
 //     ^ @boolean
   RETURN NONE
+
+// vim: set filetype=helixql: