`vim.wait()` cannot be called in a fast callback since the main loop
cannot be run in that context as it is not reentrant

Fixes #26122

Move default mappings and autocommands into a separate module and add
comments and docstrings to document each of the defaults.

Co-authored-by: Peter Aronoff <peter@aronoff.org>

fixes #18155

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`.

- Remove the usage of the term "defer" to avoid confusion with
  `vim.defer_fn`, which also calls `vim.schedule_wrap` internally.
- Explicitly state that `vim.schedule_wrap` returns a function in the
  text.
- Mention that arguments are passed along.
- Include a usage example.
- Rename param to `fn`.