Add use_snapshot()

[jonthegeek]

Implement a use_snapshot() helper to go with use_r() and use_test().

I ran out of time and didn't implement tests or NEWS for this yet (and there's a solid chance that this doesn't quite nail it), but I wanted to get it checked in.

Fixes #2156.

[jonthegeek]

This should be good to go now!

[jennybc]

@jonthegeek and/or @maelle What is the workflow in which you regularly want to open snapshot files in an editor? I mostly interact with them in the testthat::snapshot_review() app or in my git client. On the occasion I want to open a snapshot file, I think I just fall back to whatever usual ways I open files in, e.g., my IDE. It feels like there's some workflow context here that I'm missing.

[jonthegeek]

@jonthegeek and/or @maelle What is the workflow in which you regularly want to open snapshot files in an editor?

I think my original interest in this was more for the other direction, where I'm looking at a snapshot (on a rare occasion when I edited one directly for an anticipated result or something) and want to hop to the test or R file with my hotkeys. I've had situations where I want to edit the file to define what I think it should look like, though (before working out exactly how to do that), so that's where this would come in for me. Not super common but it will be nice when it comes up.

[maelle]

I was interested in the other direction compared to @jonthegeek. If I'm reviewing a new-to-me package and reading a test file that contains snapshot tests, I'd like to be able to more easily go to the corresponding snapshot files. And obviously I'd then need to go back.

If it were not use_snapshot(), I'd imagine this as an IDE feature like what allows us to click on a function to go read its definition.