feat: ignore swapfile for running Nvim processes #25336

Problem:
The swapfile "E325: ATTENTION" dialog is displayed when editing a file
already open in another (running) Nvim. Usually this behavior is
annoying and irrelevant:
- "Recover" and the other options ("Open readonly", "Quit", "Abort") are
  almost never wanted.
- swapfiles are less relevant for "multi-Nvim" since 'autoread' is
  enabled by default.
  - Even less relevant if user enables 'autowrite'.

Solution:
Define a default SwapExists handler which does the following:
1. If the swapfile is owned by a running Nvim process, automatically
   chooses "(E)dit anyway" (caveat: this creates a new, extra swapfile,
   which is mostly harmless and ignored except by `:recover` or `nvim -r`.
2. Shows a 1-line "ignoring swapfile..." message.
3. Users can disable the default SwapExists handler via `autocmd! nvim_swapfile`.

src/nvim/eval/userfunc.c

@@ -2491,7 +2491,7 @@ void ex_function(exarg_T *eap)
         } else if (line_arg != NULL && *skipwhite(line_arg) != NUL) {
           nextcmd = line_arg;
         } else if (*p != NUL && *p != '"' && p_verbose > 0) {
-          give_warning2(_("W22: Text found after :endfunction: %s"), p, true);
+          swmsg(true, _("W22: Text found after :endfunction: %s"), p);
         }
         if (nextcmd != NULL) {
           // Another command follows. If the line came from "eap" we