Add support for the stack command

[Alphare]

I am note certain what the git equivalent is, so I left it unimplemented (and hidden) for now.

Related: I think the keyboard shortcuts should be part of the config file, although we would have to warn about conflicts and unreachable chords.

[vamolessa]

Since there could be no git equivalent, couldn't this be a custom command? like, you can put this on the .verco/custom_commands.txt file:

s hg stack --color always

and then execute it with xs

[Alphare]

Well custom commands are more for unusual or... well truly custom commands I would say. I can see adding support for selecting through the stack to get the export or updateing to any given revision in the stack for example.

The hg stack command is used by pretty much all users of topic.

[vamolessa]

From what I understand, this is a mercurial extension. So people may not have that command installed. I'm not sure how we should deal with that or even if it should be added as a regular command.

Maybe this is something that could be a verco extension itself?

[Alphare]

It could be gated behind a config option like hg.topic_enabled if you're uncomfortable with it being a top-level command?