Problem:
On Windows, since b360c0608556, UI may hang on exit. #33019

Solution:
Restore the hack in on_proc_exit, until we can figure out why rpc_close
is not called in the UI client when the server closes the channel.

Problem:
`receive_msgpack()` calls `channel_close()` just before calling
`chan_close_on_err()`, even though `chan_close_on_err()` always calls
`channel_close()`.

Solution:
Remove the redunant `channel_close()` in `receive_msgpack()`.

Problem:

When a function like vim.wait() is used, we continuously drain the main
event queue until it is empty, never stopping for user input. This means
the libuv timer never runs and the terminal never gets refreshed, so
emit_termrequest continously reschedules itself onto the same event
queue, causing an infinite loop.

Solution:

Use a separate "pending" event queue, where events that require a
terminal refresh are temporarily placed. Drain this queue after a
terminal refresh and events are copied back onto the main queue. This
prevents infinite loops since the main event queue will always be able
to properly drain.

Problem:
on_proc_exit() has a special-case that assumes that the UI client will
never spawn more than 1 child process.

Solution:
If the Nvim server exits, the stream EOF will trigger `rpc_close()` in
the UI client, so we don't need the special case in `on_proc_exit`.
Pass `Channel.exit_status` from `rpc_close()` so that the correct exit
code is reflected.

Problem:
On Windows, spawning the `nvim --embed` server with `detach=true` breaks
various `tt.setup_child_nvim` tests.

Solution:
Make this behavior opt-in with an env var, temporarily.

Problem:
Cannot detach the current UI.

Solution:
- Introduce `:detach`.
- Introduce `Channel.detach`.

Co-authored-by: bfredl <bjorn.linse@gmail.com>

Co-authored-by: Dustin S. <dstackmasta27@gmail.com>
Co-authored-by: Ferenc Fejes <fejes@inf.elte.hu>
Co-authored-by: Maria José Solano <majosolano99@gmail.com>
Co-authored-by: Yochem van Rosmalen <git@yochem.nl>
Co-authored-by: brianhuster <phambinhanctb2004@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Also print stderr error messages with ErrorMsg highlight group.