Fix did:web encoding/decoding

[matthieusieben]

Current implementation does not properly encode/decode did webs as per spec:

https://w3c-ccg.github.io/did-method-web/#read-resolve

[devinivy]

A change like this seems like a good opportunity to fill out some unit tests. Could we add some as part of this?

[matthieusieben]

Good point. I added some tests in.

[bnewbold]

didn't see this when it first went by, but note that we have a bunch of atproto-specific restrictions on did:web in the specs:

In the context of atproto, only hostname-level did:web DIDs are supported: path-based DIDs are not supported. The same restrictions on top-level domains that apply to handles (eg, no .arpa) also apply to did:web domains. The special localhost hostname is allowed, but only in testing and development environments. Port numbers (with separating colon hex-encoded) are only allowed for localhost, and only in testing and development.

https://atproto.com/specs/did

I'm pretty confident about those restrictions, and that we don't want to support the full did:web functionality. I don't have a direct citation, but some W3C DID folks have also expressed that path-based did:web isn't particularly safe or recommended, and that did:web is still just a draft, so there isn't very strong pressure to implement it exactly as described in the draft doc.