[RFC] Vue 3 support

[ShGKme]

This is the last library missing Vue 3 support.

According to the GitHub search, it is used in:

• As pure API:
  • nextcloud/spreed
  • pulsejet/memories
  • LibreSign/libresign
• Also as a Vue component:
  • nextcloud/server/apps/files
  • nextcloud/photos

I see 3 options:

1. Migrate the library to Vue 3
   • Breaking change: requiring a new major release
   • Has problems described in [RFC] export of vue components #1809
   • 1-year-old draft: Migrate to Vue 3 #1197
2. Split: [RFC] export of vue components #1809
3. Intermediate solution: split only internally
   • Internal preparation to option 2
   • Non-breaking change
   • Works for API users
   • Doesn't work for Vue 3 migration of nextcloud/server and nextcloud/photos
   • Blocks neither option 1 nor option 2
   • feat: cheap Vue 3 support via separating core login and ui by package exports #1811

In general, I'd prefer to go with option 2, as @susnux proposed.

In the meanwhile I'd propose the option 3 as a preparation for 2, that also fixes Vue 3 support for some apps.

[ShGKme]

• Discussed with @susnux
• It was decided to go with [RFC] export of vue components #1809

[susnux]

It was decided to go with #1809

If we do so we can also think about deprecating this package in general and move the uploader API to @nextcloud/files/uploader. This reduces the number of packages we need to maintain and the API is very similar to what is already available in @nextcloud/files.