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/funcs.c

@@ -8236,7 +8236,7 @@ static void f_swapfilelist(typval_T *argvars, typval_T *rettv, EvalFuncData fptr
 static void f_swapinfo(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 {
   tv_dict_alloc_ret(rettv);
-  get_b0_dict(tv_get_string(argvars), rettv->vval.v_dict);
+  swapfile_dict(tv_get_string(argvars), rettv->vval.v_dict);
 }
 
 /// "swapname(expr)" function