feat: support pull requests

Support for pull requests, it's not standard in NIPs yet[1] but there must be
two clients that support it to be included in NIPs, n34 is one of them.

This support:
- Create a new PR `n34 pr new`
- Update PR tip `n34 pr update`
- View a PR `n34 pr view`
- List PRs `n34 pr list`
- Add `PR` and `PR-update` kinds as roots in `NostrClient::find_root`
- Support issuing statuses for PRs
- Support GRASP PR and PR-update
- Update the docs

[1] https://github.com/nostr-protocol/nips/pull/1966

README.md

@@ -25,7 +25,7 @@ Check the documentation at [n34.dev]
 - [X] Issues (Send, view and list)
 - [X] Replies
 - [X] Issues and patches status
-- [ ] Pull requests ([nostr-protocol/nips#1966])
+- [X] Pull requests ([nostr-protocol/nips#1966])
 - [X] Gossip Model ([NIP-65])
 - [X] Proof of Work ([NIP-13])
 - [X] `nostr:` URI scheme, in the issue/reply content ([NIP-21])