[RFC] Vue 3 support

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 https://github.com/nextcloud-libraries/nextcloud-upload/issues/1809
   • 1-year-old draft: https://github.com/nextcloud-libraries/nextcloud-upload/pull/1197
2. Split: https://github.com/nextcloud-libraries/nextcloud-upload/issues/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
   • https://github.com/nextcloud-libraries/nextcloud-upload/pull/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.

Start by reviewing the three migration options, especially issue #1809 and the referenced draft PR #1197. Compare the proposed intermediate approach with PR #1811 and the listed Nextcloud consumers; done means an agreed Vue 3 support path that preserves the intended compatibility and migration constraints.