Add surrounding_pairs option to xi-core-lib; add InsertDrift

[lord]

Summary

This adds the surrounding_pairs option. If all selections are regions (and not carets) and the user types a first half of a surrounding pair, then we will surround the selection with the surrounding pair instead of replacing the selection. This config option can be edited on a per- language basis.

This change also adds InsertDrift as a replacement for the old keep_selections option. The keep_selections bool, if true, overrode the 'after' property, ensuring that inserts 'drifted' to the side of a cursor that was inside a selection. This change keeps that same functionality, but adds the option to have inserts 'drift' to the outside of a selection instead.

It's not ~fully tested or complete, but the functionality works, and I thought I'd submit this WIP just to see if this is the right approach before investing more time into it?

Related Issues

A part of several tasks within #647

Checklist

• Still need to add tests

Review Checklist

• I have responded to reviews and made changes where appropriate.
• I have tested the code with cargo test --all / ./rust/run_all_checks.
• I have updated comments / documentation related to the changes I made.
• I have rebased my PR branch onto xi-editor/master.

[cmyr]

Cool, I'll put this in my review docket for tomorrow. 👍

[lord]

@cmyr added tests!

[cmyr]

Okay, I'm into this; I think there are some other little problems with how we deal with selection expansion but this is a clean patch that fixes a real problem, so let's get this in and see how much it buys us!

I think we need a rebase, but otherwise looks good. You should have commit writes to this repo, and can merge whenever.

Playing with this, I think it naturally makes sense to look at the sibling feature of 'auto-closing pairs'?

[lord]

Thanks, sounds good! Ok to use GitHub's "rebase and merge" button, or do we prefer manual rebases?

[lord]

And sounds good — glad to look at auto-closing pairs next.

[cmyr]

@lord actually rebase and merge should be fine; i was thinking of a manual rebase to get the updated CI config but it's already passed travis so I think we're gtg :)

[lord]

Done! 😄