Over-indent multiline interpolations

[dralletje]

I made this PR after needing the same features as #12

It looks at the value in interpolations and adds the indentation to every but the first line (because that one already has the indentation).

I'm not sure if you'd want this in the package, as it assumes a lot more stuff, but on the other hand I can not imagine a scenario where you don't want this 🙃

[dralletje]

Added another commit to make a bit more strict

[rattrayalex-stripe]

cc @dmnd is this mergeable?

[ZhouHansen]

Hi! @rattrayalex-stripe you can use endent to achieve what you want before this pr merged.

[JoshuaKGoldberg]

Note: I posted a question in #12 about whether we want this to always be the case, or an opt-in.

[JoshuaKGoldberg]

Closing for now to keep the queue small - let's discuss in #12. Cheers!