Migrate vue components to setup (#32329)

Migrate vue components to setup (#32329)

Migrated a handful Vue components to the `setup` syntax using
composition api as it has better Typescript support and is becoming the
new default in the Vue ecosystem.

- [x] ActionRunStatus.vue
- [x] ActivityHeatmap.vue
- [x] ContextPopup.vue
- [x] DiffFileList.vue
- [x] DiffFileTree.vue
- [x] DiffFileTreeItem.vue
- [x] PullRequestMergeForm.vue
- [x] RepoActivityTopAuthors.vue
- [x] RepoCodeFrequency.vue
- [x] RepoRecentCommits.vue
- [x] ScopedAccessTokenSelector.vue

Left some larger components untouched for now to not go to crazy in this
single PR:
- [ ] DiffCommitSelector.vue
- [ ] RepoActionView.vue
- [ ] RepoContributors.vue
- [ ] DashboardRepoList.vue
- [ ] RepoBranchTagSelector.vue