Integrate Context UI into Duo Chat App.vue - VSCode - Injected Context

Pull in the gitlab/ui changes for pinned context UI to the VSCode Duo Chat webview. Integrate the new context-items-menu into chat, and integrate with language server events/data.

The last gitlab/ui issue in the chain is gitlab-ui!4581 (merged) so this issue cannot be completed until that MR is merged and the gitlab/ui version it is released as is pulled into VSCode.