enhance(oauth): pre-register a builtin OAuth2 application for gitea-vscode (#39785)

The [Gitea VS Code extension](https://gitea.com/gitea/gitea-vscode)
currently signs in through the builtin `tea` OAuth2 application. Gitea
keeps one grant per user and application, so anyone who has already
signed in with the tea CLI gets `a grant exists with different scope`
when the extension requests its own scopes, and sign-in fails
(gitea/gitea-vscode#24).

This adds a dedicated builtin application for the extension, following
the same mechanism as the existing CLI entries:

custom/conf/app.example.ini

@@ -638,7 +638,8 @@ ENABLED = true
 ;; * https://github.com/git-ecosystem/git-credential-manager
 ;; * https://gitea.com/gitea/tea
 ;; * Gitea App (the official Gitea mobile app)
-;DEFAULT_APPLICATIONS = git-credential-oauth, git-credential-manager, tea, gitea-app
+;; * https://gitea.com/gitea/gitea-vscode
+;DEFAULT_APPLICATIONS = git-credential-oauth, git-credential-manager, tea, gitea-app, gitea-vscode
 ;;
 ;; By default, OAuth2 applications can only use "http" and "https" as their redirect URI schemes.
 ;; If you need to use other schemes (e.g. for desktop applications), you can specify them here as a comma-separated list.