[UPDATE][VIM] Factorise b:undo_ftplugin

Instead of re-writing the same condition each time, I put the function
in an autoload plugin instead (which is going to be loaded just about
every time I start writing code, but that's neither here nor there) to
stop repeating myself.

vim/.vim/autoload/ftplugined.vim

@@ -0,0 +1,6 @@
+" Create the `b:undo_ftplugin` variable if it doesn't exist
+function! ftplugined#check_undo_ft()
+    if !exists("b:undo_ftplugin")
+        let b:undo_ftplugin=''
+    endif
+endfunction