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`.

runtime/doc/vim_diff.txt

@@ -139,6 +139,11 @@ nvim_terminal:
 nvim_cmdwin:
 - CmdwinEnter: Limits syntax sync to maxlines=1 in the |cmdwin|.
 
+nvim_swapfile:
+- SwapExists: Skips the swapfile prompt (sets |v:swapchoice| to "e") when the
+  swapfile is owned by a running Nvim process. Shows |W325| "Ignoring
+  swapfile…" message.
+
 ==============================================================================
 New Features						       *nvim-features*