fix(treesitter): enforce lowercase language names (#28546)

* fix(treesitter): enforce lowercase language names

Problem: On case-insensitive file systems (e.g., macOS), `has_parser`
will return `true` for uppercase aliases, which will then try to inject
the uppercase language unsuccessfully.

Solution: Enforce and assume parser names to be lowercase when
resolving language names.

runtime/lua/vim/treesitter/language.lua

@@ -88,6 +88,9 @@ function M.add(lang, opts)
     filetype = { filetype, { 'string', 'table' }, true },
   })
 
+  -- parser names are assumed to be lowercase (consistent behavior on case-insensitive file systems)
+  lang = lang:lower()
+
   if vim._ts_has_language(lang) then
     M.register(lang, filetype)
     return