feat: ":write!" skips "file changed" warning #18665

Problem:
Cannot opt-out of "WARNING: The file has been changed since reading
it!!!", even with ":write!".

Solution:
Change ":write!" to skip the warning.

closes #7270

runtime/doc/vim_diff.txt

@@ -372,6 +372,7 @@ Lua interface (|lua.txt|):
 Commands:
   |:doautocmd| does not warn about "No matching autocommands".
   |:wincmd| accepts a count.
+  `:write!` does not show a prompt if the file was updated externally.
 
 Command line completion:
   The meanings of arrow keys do not change depending on 'wildoptions'.