feat: add hg implementation

[zegervdv]

As the title says, adds mercurial (hg) support.

I stubbed those extra attributes like i_crlf and attr as they don't seem to be used right now, and I'm not sure if and how to get that information from mercurial.

I need to set the config to vcs = 'hg' for this to work, but would it be an option to make it figure that out automatically?
I guess that could also be done in the on_attach function by using the client's root dir.

[joechrisellis]

Thanks so much for writing this. 🙏

I'll block some time out this weekend for a proper review and test. 👍

[joechrisellis]

@zegervdv apologies for the delay on this. Had a chance to check this out today and all seems to work great. Given that hg is less popular than Git, I'm happy to merge this in with the TODOs and shake out any bugs if/when they show up. Thanks for your work here!

[zegervdv]

No apologies needed, it's your spare time :).