Implement new textobject for indentation level

[EpocSquadron]

Closes #1810.

Current Implementation

This implements a textobject corresponding to the current indentation level of the selection, bound on i. It is only implemented for the "match mode" (eg. mii or mai). It takes a count, where count extends the selection to further leftward indented surroundings than represented by the current selection. mai will skip over empty lines to extend the selection, whereas mii will stop on the first empty line above or below. Selection direction is preserved on extension.

I've added as many tests as I could keep straight in my head. I almost feel like some sort of property testing might be better able to find the edge cases, but what is implemented is what I had time to think through.

Future Direction

I could implement "next" (]i) and "prev" ([i) to be the next contiguous block of the same indentation level as the current selection, after one or more blank lines. However I don't want to hold this implementation back as it is now complete and I have little time to dedicate to further features. Others are welcome to implement, or I may return to it at a later date.

[EpocSquadron]

I've updated the original description to hopefully be more readable, included a screencast of the feature, and made clear the call for feedback on potentially extending the logic.

[daedroza]

Hey @EpocSquadron , this look like a great change, for next and prev implementation, only the contiguous block should matter. I would be waiting for that change if you're willing to implement, thanks again!

[EpocSquadron]

I've updated this branch, squashed the changes, addressed feedback, and updated the description of the PR to reflect the current state. I removed the short screen recording I had up prior as it no longer accurately reflected the functionality. I ran out of time to record a new one.

@daedroza I'm not able to dedicate time towards the next/prev motions at the moment. I may return at a later date but no promises or timeline. I did at least move forward with distinguishing the inside and around versions, so hopefully that is sufficient to get this merged and expand on later.