feat: add undo!

Allows using `undo!` to undo changes and remove them from the undo-tree. Can only be used for moving backwards in the same undo branch.

src/nvim/ex_cmds.lua

@@ -2947,7 +2947,7 @@ module.cmds = {
   },
   {
     command='undo',
-    flags=bit.bor(RANGE, COUNT, ZEROR, TRLBAR, CMDWIN),
+    flags=bit.bor(BANG, RANGE, COUNT, ZEROR, TRLBAR, CMDWIN),
     addr_type='ADDR_OTHER',
     func='ex_undo',
   },