refactor: moved issuable into the `vue` sub-folder

This is the preparation step to account for more webviews powered by Vue but still managing the dependencies only at the top webviews/vue level.

This is the very first step in the multi-step work to add GitLab Duo Chat to the GitLab Workflow extension in VSCode and Web IDE. The next steps will be:

• rename webview_controller to issuable_controller as that's what it really is
• adding GitLab Duo Chat webview
• connect that webview to build and watch mechanisms
  • a sub-task here is to abstract some of the helpers for issuable_controller and gitlab_chat_controller functions into the platform to decouple the controllers from any particular implementation of things like readFileSync, path, etc. to make webviews universal and not desktop- or browser-only.