specs: allow digits in DID method

[bnewbold]

Allowing digits does seem to be correct upstream:
https://www.w3.org/TR/did-core/#did-syntax

and there are methods like did:3 and did:web7 which use them:
https://w3c.github.io/did-spec-registries/#did-methods

As part of this change we should:

• update the specs and examples
• update the shared interop test vectors
• update golang SDK (indigo)
• update typescript SDK (atproto)
• dev comms to other promoted SDK maintainers

Feels like a bit of a bummer that they allow this as it visually blurs with host:port style syntax, but I guess the required second colon disambiguates.

cc: @matthieusieben

[matthieusieben]

Note that the host:port must be url encoded in did:web, which means that the colon : as port separator is never really visible anyway..

[bnewbold]

what I meant was the kind of visual/superficial confusion between:

https://did:8080/app.bsky.feed.post/abc123
at://did:80:80/app.bsky.feed.post/abc123

I think it is a tiny bit less confusing if it had to be:

at://did:eighty:80/app.bsky.feed.post/abc123