Support quitting with exit code

[cole-h]

Describe your feature request

In kakoune, you can return any error code you want when quitting via :quit [exit code]. One example of where I use this functionality a lot is when I write a git commit message, save the buffer, but then realize there's something else I need to do (rather than accepting and then git reset --soft HEAD^).

One implementation detail would be what we want to do when there's more than 1 view (since :q really just closes views). Likely just return an error.

[sudormrfbin]

Vim has a separate :cquit command which I use with git commit regularly:

:cq[uit][!]
:{N}cq[uit][!]
:cq[uit][!] {N}     Quit Vim with error code {N}.  {N} defaults to one.
                    Useful when Vim is called from another program:
                    e.g., a compiler will not compile the same file again,
                    `git commit` will abort the committing process, `fc`
                    (built-in for shells like bash and zsh) will not
                    execute the command, etc.
                    {N} can also be zero, in which case Vim exits
                    normally.
                    WARNING: All changes in files are lost.  It works like
                    ":qall!" |:qall|, except that Nvim exits non-zero or
                    [count].

[cole-h]

I actually like that better. I was originally going to augment the existing :qa commands, but a dedicated command seems just right. Implemented in #1096.