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)

plugin/filetypes.lua

@@ -20,7 +20,6 @@ local filetypes = {
   godot_resource = { 'gdresource' },
   haskell = { 'hs' },
   haskell_persistent = { 'haskellpersistent' },
-  helixql = { 'hx' },
   idris = { 'idris2' },
   ini = { 'confini', 'dosini' },
   janet_simple = { 'janet' },