Import/export bookmarks as JSON

[Wikid82]

Problem

Bookmarks live only in VS Code's per-workspace `workspaceState` (`BookmarkStore`). There's no way to back them up, move them to a new machine, or share a curated bookmark set with a teammate onboarding to the same multi-repo workspace.

Proposal

Add two commands:

• `workspace-file-bookmarks.exportBookmarks` — writes bookmarks + folders to a JSON file the user picks via `showSaveDialog`.
• `workspace-file-bookmarks.importBookmarks` — reads a JSON file via `showOpenDialog` and merges (or replaces, with confirmation) into the current workspace's store.

Notes

Also serves as a stopgap for #cross-workspace-sync — export/import between workspaces is simpler to ship than true sync.