NSID syntax: formalize fragment and glob variations

[blowdart]

The nsid spec mentions that a variation can contain an appended fragment and * as a glob character.

Neither appear in any of the valid test cases so it's unclear if we need to support them right now or not.

[bnewbold]

In both cases, I don't think we've fully formalized the grammar/restrictions yet. Eg, for fragments, what exact characters can show up in the fragment? For globs, do we support arbitrary string globs (app.bsk*), or only at a period position (app.bsky.*), can they be positional (app.*.feed.post), and are they recursive or does ** specify recursive.

The fragment syntax is used the in the wild for Lexicon references, so we should probably get that nailed down.

This repo isn't a bad place to file this issue, but we have just moved the interop test files over to https://github.com/bluesky-social/atproto-interop-tests, and the specs themselves are versioned in https://github.com/bluesky-social/atproto-website.

I'm going to move this issue over to the specs repo.