feat: add goto_first_selection, goto_last_selection

[Axlefublr]

The usecase is to have made some selections, and then jumping to either the first or the last one, to then "remove all other selections" on. Very helpful for navigation!

[the-mikedavis]

We aren't always rigorous/consistent about what verb we use for selection changes ("goto" vs. "move" for example) but here "goto" seems off to me since it's usually used for changing the selection range (and sometimes file), like goto_definition. I'm not sure what the right verb is for this though. "Rotate" also doesn't seem quite right but it would match rotate_selections_{forward,backward}. It's a bit wordier but how about set_{first,last}_primary_selection?

[Axlefublr]

hmmm yeah good catch. goto is off here
set_first_primary_selection incurs the natural question of "to what?", as if you're setting the primary selection to something. I think set_primary_selection_first would work a bit better
I'm on the side of rotate_selection_first, though. it's imperfect too, but I think if a user knows what rotate_selections_forward means (I didn't get it at all until I tried it), they should be able to guess what _first would mean.
I'll change to rotate_selection_… format, and hear back your thoughts; I'm not necessarily tied to this variant

[Axlefublr]

as I touched upon, I don't quite like rotate_selections to begin with as a wording choice, but I think it's better for consistency now to stick to it, so that primary selection-changing actions are all together, and more discoverable to the user