fix: do not save K_EVENT as lastc in Insert mode

I'm also gonna move some other K_EVENT-related tests to vim_spec.lua

src/nvim/edit.c

@@ -643,7 +643,10 @@ static int insert_check(VimState *state)
   update_curswant();
   s->old_topline = curwin->w_topline;
   s->old_topfill = curwin->w_topfill;
-  s->lastc = s->c;   // remember previous char for CTRL-D
+
+  if (s->c != K_EVENT) {
+    s->lastc = s->c;  // remember previous char for CTRL-D
+  }
 
   // After using CTRL-G U the next cursor key will not break undo.
   if (dont_sync_undo == kNone) {