[selectors-4] Replace <an+b> with <an-b>

[cdoublev]

Fixes #9494.

Depends on #9480.

[fantasai]

A few comments but also a question about bikeshedding: as we're getting further away from the actual form of the notation "2n+1", it's getting more confusing to refer to the notation. An+B was somewhat understandable, because it pattern-matches against the notation. But <an-b> just seems like weird 1970s computer garbage naming.

Some possible replacements:

• <An-B> at least helps it parse as three components (a, n, b), rather than two (an, b).
• <ith> or <index> or something like that
• other ideas?

UPDATE: Filed the bikeshedding into #9473 (comment)

[cdoublev]

Marking this PR as a draft while there are discussions to rename <an-b>.