FR: Add support for `@-<n>` syntactic sugar

[zx8]

Currently, to reference a previous commit N revisions ago, I use jj show @----- because it's quick to type.

It would be neat if show @-5 (or similar) was supported, to save the additional keystrokes, similar to git's git show @~5 syntax.

[scott2000]

This was also discussed in #3337, where we ended up adding parents(x, n) instead (there is now also first_parent(x, n), which matches Git's behavior more closely). If that's too long, one workaround could be to define shorter aliases for parents(x, n) or first_parent(x, n) such as:

[revset-aliases]
"p(x, n)" = "first_parent(x, n)"

This would allow p(x,5) to be equivalent to x~5 in Git syntax.

I'm not opposed to adding syntax for this though.

[yuja]

Also note that -/+ cannot be used as an infix operator because we allow symbol-name-like+this. There aren't many characters that are easy to type and require no shell quoting.