Rename to g:loaded_fern from g:fern_loaded

Author: lambdalisue

plugin/fern.vim

@@ -1,7 +1,8 @@
-if exists('g:fern_loaded')
+if exists('g:loaded_fern')
   finish
 endif
-let g:fern_loaded = 1
+let g:loaded_fern = 1
+let g:fern_loaded = 1 " Obsolete: For backward compatibility
 
 command! -bar -nargs=*
       \ -complete=customlist,fern#internal#command#fern#complete