feat(treesitter): add injection language fallback (#24659)

* feat(treesitter): add injection language fallback

Problem: injection languages are often specified via aliases (e.g.,
filetype or in upper case), requiring custom directives.

Solution: include lookup logic (try as parser name, then filetype, then
lowercase) in LanguageTree itself and remove `#inject-language`
directive.

Co-authored-by: Lewis Russell <me@lewisr.dev>

test/functional/treesitter/parser_spec.lua

@@ -783,7 +783,7 @@ int x = INT_MAX;
         return list
         ]]
 
-        eq({ 'gsub!', 'inject-language!', 'offset!', 'set!', 'trim!' }, res_list)
+        eq({ 'gsub!', 'offset!', 'set!', 'trim!' }, res_list)
       end)
     end)
   end)