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.lua

@@ -11123,8 +11123,8 @@ M.funcs = {
       	user	user name
       	host	host name
       	fname	original file name
-      	pid	PID of the Vim process that created the swap
-      		file
+      	pid	PID of the Nvim process that created the swap
+      		file, or zero if not running.
       	mtime	last modification time in seconds
       	inode	Optional: INODE number of the file
       	dirty	1 if file was modified, 0 if not