change PortableDid's did property to uri

[mistermoe]

PortableDid is returned by every DidMethod.create implementation. e.g.

const did: PortableDid = await DidDhtMethod.create({ publish: true })

when writing code where you want to use the actual did's URI, it ends up looking something like this:

const vc = await VerifiableCredential.create({
  type    : 'TBDeveloperCredential',
  subject : did.did,
  issuer  : did.did,
  data    : {
    username: 'moegrammer'
  }
})

did.did can be a bit confusing. i think changing did.uri would make things a bit more clear with respect to when someone is referring the string representation of a did

[mistermoe]

web5-kt and web5-rs have snapped to using uri