Support Remix projects without Utopia config / routes

[liady]

A vanilla project with an added remix npm package should show the pages tab (continuation of #5410), even if there are no pages configured yet.

• Show the pages tab based on some condition (eg: @remix being imported and / or routes existing) that works in a project without a .utopia config file (e.g. @remix package being imported, routes existing etc)

If that tab requires Utopia config to be shown, that shouldn't be the case and is faulty behaviour. Eliminate it:

• if 'add page' requires page template:
  • disable the button if the template does not exist, or there is no reference to one
  • for the Remix starter project, create two templates and add them to the config
• if Favorites requires an entry in the Utopia config, make it work without it and then add the entry (creating the config file if needed)

if the whole tab contents still can't be shown:

• add a corresponding failure state into the tab: Pages are not available in this project. You can add pages support by adding routes via the file browser (docs)
• If we have additional info about the reason, add it below the message