Add g:loaded_matchit check (#13226)

I have added g:loaded_matchit check to skip matchit loading like other default plugins.

runtime/plugin/matchit.vim

@@ -1,4 +1,4 @@
 " Nvim: load the matchit plugin by default.
-if stridx(&packpath, $VIMRUNTIME) >= 0
+if !exists("g:loaded_matchit") && stridx(&packpath, $VIMRUNTIME) >= 0
   packadd matchit
 endif