feat: allow both OA and W3C VCs to be digested or signed

[HJunyuan]

What does this PR do?

This PR will introduce breaking changes to OA v4 (which is currently in beta).

1. Allow both OA and W3C VCs to be digested or signed
2. Use "digest" terminology instead of "wrap"
3. signVc() will automatically digest before performing signing. As such, digest and sign are now separate processes
   1. signVc() will not re-sign if proof object is already defined
4. digestVc() is left available for use with Ethereum issuance
5. Stop grouping or the usage of any shared functions/utils between v2 and v4 (e.g. src/index.ts)
   1. In future, those functions will only serve v2 once v3 has been removed
6. Rename all remaining mention of document to vc