Add new /instance<instance-id>/editor view

[joepavitt]

As part of #3646, we need to create a new view that will host the "Immersive" experience.

Considerations:

• Layouts: Every page on our application utilises the /src/layouts/Platform.vue layout component, which is defined at the very top App.vue level. This will be the first view we have that does not need the very top-level navigation header that this view provides. We already have a isModalPage check in App.vue, we could extend this to handle this particular page.
• Editor Availability: We need to have a temprorary solution in place for when the Editor is not available. As an extensin of thr work, we will eventually implement Improved feedback of Node-RED editor status with integrates editor #3655, but for now we should offer a short-term solution.
• Tabs: We pass :tabs="navigation" into the instance/index.vue, which utilises the full ff-page-header (SectionNavigationHeader.vue) component. For this new view, we only need the tabs part of that ff-page-header, which can be achieved with <ff-tabs v-if="tabs" :tabs="tabs" />.
• Reusable Pages: Each child page of /instance should already be available in standalone components, and so these should be re-usable within the new /editor view.
• Navigation/URL Structure: Something we need to settle on is how we're going to structure the new URLs here. Currently, each tab navigates to a completely new view, now each tab should modify new child <router-view> of the /editor page. I think it's a good first step to have /instance/<instance-id>/editor/snapshots or something along those lines, which would mimic the /instance/<instance-id>/snapshots view.

Acceptance Criteria:

• User can navigate directly to the /editor URL to view this new experience
• The bottom half view (with tabs) does not need to be resizable (that's covered in Optimize the Node-RED immersive editor experience #3657) - instead a single chevron on the right-side would enable a minimise/maximise approach