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

docs/index.html

@@ -375,8 +375,8 @@
                 <span class="feature-text">Issues and patches status</span>
               </li>
               <li>
-                <span class="feature-check pending"></span>
-                <span class="feature-text pending">Pull requests (<a href="https://github.com/nostr-protocol/nips/pull/1966" target="_blank" rel="noreferrer">nostr-protocol/nips#1966</a>)</span>
+                <span class="feature-check completed"></span>
+                <span class="feature-text">Pull requests (<a href="https://github.com/nostr-protocol/nips/pull/1966" target="_blank" rel="noreferrer">nostr-protocol/nips#1966</a>)</span>
               </li>
             </ul>
           </div>