feat: introduced support for Vue2 webviews

This MR introduced the basis for adding the webviews, powered with Vue 2.

This is especially important for the apps, using @gitlab/ui. It also makes the path for applications from the main gitlab product easier.

Full disclosure: this MR is a preparation step for moving the gitlab_duo_chat webview into Vue 2 supported environment in the context of Shared UI for GitLab Duo chat (&11263). As an end-goal, gitlab_duo_chat will end up being a @gitlab/ui to DRY out the component so it could be used everywhere (incl. Web UI) and be updated simultaneously in all environments.

Related to #993 (closed)