feat(github): pull request tool window, timeline and review flow (#477)

Replaces the Pull Requests tool window with a full review workflow on top of gh's authentication.

- List with search, quick filters and paging; a tab per pull request and a New Pull Request tab.
- Details: commit picker, changes tree with viewed state, CI and reviewer status, merge, close and reopen.
- Timeline editor: comments, review threads, reactions and suggested changes.
- Review in the diff viewer and, for the checked-out branch, in the editor (review mode).
- New generic vcs extension points: registerDiffViewerProvider, lineInlays, reviewGutter, branch widget extensions; new vcs_merge_base and gh_api commands.
- CSP: img-src allows https://avatars.githubusercontent.com for avatars.

Fixed while testing against a real repository: remotes behind an SSH host alias, mergeability stuck on UNKNOWN, Apply Locally staying disabled after a checkout, relative links navigating the app's page, and review inlays wider than the pane.

Co-authored-by: imacross <[email protected]>

src/features/vcs/diffNavigator.ts

@@ -1,4 +1,5 @@
 /** The focused diff editor's chunk navigation, for actions that must not load the diff editor. */
+import type { DiffViewerHost } from "./diffViewerProviders";
 
 export interface DiffNavigator {
   next(): void;
@@ -10,6 +11,8 @@ export interface DiffNavigator {
   file(dir: 1 | -1): void;
   /** The viewer's settings as a popup menu (⇧⌘D). */
   showSettings(): void;
+  /** The built panes, for diff viewer contributions' actions (null while loading). */
+  viewer?(): DiffViewerHost | null;
 }
 
 let focused: DiffNavigator | null = null;