refactor(parser): expose get_lang_from_ft as public API

Problem: `get_lang_from_ft` is needed by the gitsigns module but was
a local function in `parser.lua`, causing duplication.

Solution: assign it to `M.get_lang_from_ft` for reuse by other modules.

lua/diffs/parser.lua

@@ -353,6 +353,8 @@ function M.parse_buffer(bufnr)
   return hunks
 end
 
+M.get_lang_from_ft = get_lang_from_ft
+
 M._test = {
   ft_lang_cache = ft_lang_cache,
 }