Add new basic registry explorer app

What does this MR do?

This MR is part of a series of merge requests to bring new UX and routing methods to the Container Registry. This enables the Container Registry pages to behave like an SPA using vue-router.

This MR is part two which lays the ground work for the pages and includes: the mounting of the new components, the Vuex store, the router, feature flag and tests for the store. It also includes two stub components that will act as the list and details pages but they are not implemented as part of this MR.

This new UX is behind the vue_container_registry_explorer feature flag so can easily be reverted. No modifications have been made to the existing Container Registry code as part of this.

Related MRs:

• Add pagination and new routing for docker container registry
• [This MR] Add new basic registry explorer app
• Add final List and Details page for container registry explorer
• Add vue powered breadcrumb compatible with vue router

Original proof of concept MR (not to be merged): !23079 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Part of #23315 (closed)