Add status.tree_view option

[ceuk]

Hello :)

Bit of a POC really to show how #1695 might work in order to get some feedback/direction.

I think it'd also be good to be able to stage/unstage entire sections of files. Will look at that shortly, trying to squeeze this in around my work/family commitments

Let me know if this is going in the right direction anyway

[CKolkey]

While it's not something I would personally use, I know people like their tree views for these things, so I won't object to including it.

Code wise, I think the implementation is actually quite nice! I'll give it a more in-depth look over soon. Just a minor thing - prefer snake_case to camelCase for identifiers, to be consistent with the rest of the code-base.