also filter pull requests by assignee

[williamjaackson]

The Pull Requests view cannot filter or group PRs by assignee. Author and review filters cover different roles: a PR can be assigned to someone who neither authored it nor has a pending review.

Let users narrow the list to one person's assigned work and, separately, group PRs by assignee to see how work is distributed.

• Filter by an assignee's login, with "Assigned to me" and "Unassigned" options.
• Combine the assignee filter with the existing project, state, label, and review filters.
• Consider grouping by assignee as a separate follow-up.

The smallest useful scope is assignee filtering for GitHub in the web Pull Requests view, which desktop also uses. Apply it through GitHub search so it includes PRs beyond the loaded page. GitHub supports assignee:LOGIN, assignee:@me, and no:assignee.

Only offer the filter for hosts that can apply it. The current list rows do not carry assignees, so filtering already-loaded rows cannot provide this behavior for other hosts. Mobile does not currently have this PR list.

Reposted from #14334 following maintainer triage. The request was confirmed as a real gap and not a duplicate. Discussion #11134 covers negative and multi-select PR filters; #11067 proposes an Issues panel.