ref #15440

After a process's refcnt is decremented to zero, it enqueues a
`process_close_event` on its own event queue. In `process_wait`, this
event should be processed immediately so that any process close
callbacks are executed before `process_wait` returns.

Update `process_wait` to always process the process's event queue after
the process is freed, rather than the event queue passed in as an
argument.

Problem:    Code to handle callbacks is duplicated.
Solution:   Add callback_T and functions to deal with it.
https://github.com/vim/vim/commit/3a97bb3f0f8bd118ae23f1c97e55d84ff42eef20

Port Vim's put_callback() as callback_put()
because Neovim's naming convention is {type}_{action},
not {action}_{type}.

Renaming put_callback type as PutCallback.
https://neovim.io/develop/style-guide.xml#Type_Names

[skip ci]

Before #12937, the only way to specify the `$TERM` for a pty job was
through the `TERM` key in the job's opts dict.  This was shuttled to the
child process throug a special field on the PtyProcess object and
injected into the environment after forking.

Now that we have a proper way to specify the environment for a job, we
can simply ensure that the env dict has a proper `TERM` set and avoid
the extra shuttling of data around.

This deprecates the use of the `TERM` option, but will still honor it if
present, although at a lower priority than a `TERM` present in the env
dict.

This also fixes #13874 because we're no longer trying to overwrite
`TERM` in the env dict with the special pty `term_name`.  Doing so
raises an internal error because of the existing key which, under
certain circumstances, would cause the "hit enter" prompt.  However,
since the child process had already forked, there was no way for the
user to acknowledge the prompt and we would just hang there.

When starting a pty job, there are certain env vars that we need to
either add or remove.  Currently, there are two relevant scenarios.

* Removing irrelevant env vars on Unix, mostly related to the terminal
  hosting nvim since they do not apply to a libvterm-hosted terminal.

* Adding required env vars for Windows jobs.

Co-authored-by: Matthieu Coudron <mattator@gmail.com>