Avoid NeoVim error when opening .mll files

Opening a .mll file results in:
`ocamllsp: -32600: unsuppported file extension`

Signed-off-by: Edwin Török <[email protected]>

Author: edwintorok

ocaml-lsp-server/src/document.ml

@@ -8,7 +8,7 @@ module Kind = struct
 
   let of_fname_opt p =
     match Filename.extension p with
-    | ".ml" | ".eliom" | ".re" -> Some Impl
+    | ".ml" | ".eliom" | ".re" | ".mll" | ".mly" -> Some Impl
     | ".mli" | ".eliomi" | ".rei" -> Some Intf
     | _ -> None
   ;;