feat: add the 'oldest sort method to sort by creation day, ascending

[ispringle]

This adds the 'oldest keyword to sort by creation day, ascending (oldest first). This also adds a test for this behavior and the needed documentation.

There is a slightly odd behavior, as documented in the test. Because persp-find-some defaults to returning the first in the list, and with 'oldest that's the oldest, when a persp is killed it can end up going back to main instead of the previously created persp. Not sure how to handle this since technically it fits with the expected behavior of the other sort methods. I could make it so that persp-find-some will either recognize 'oldest and instead call the name/sort function with either created or accessed. Accessed makes most sense to me, but that's a design decision that I think is beyond the PR maybe, or at least beyond me since it'd be a change in how things function.

[gcv]

My instinct is that persp-find-some could be made smarter. I took a look at it, and it’s one of the least updated pieces of Perspective code. So the drawback of touching it is that it will need more thorough testing. Up to you if you want to make another PR with that change.

Meanwhile, I’ll merge this in. Thank you for contributing to Perspective.