feat: introduce useIsMobile and useIsFullscreen composables

[ShGKme]

Both composables store and return reactive state in module-scope (aka global) and are updated on resize.

Old mixins are now using new composables and are marked deprecated.

Fully compatible with Vue 3

Usage example in Options API:

<script>
import { useIsMobile } from '@nextcloud/vue/dist/Composables/useIsMobile.js'

export default {
  setup() {
    return {
      isMobile: useIsMobile(),
    }
  },

  watch: {
    isMobile() {
      // handle
    },
  },
}
</script>

<template>
  <div v-if="isMobile">I'm on mobile</div>
</template>

🚧 Tasks

• Add useIsMobile composable, that stores and returns isMobile state in module-scope (aka global)
• Add composables to build
• Refactor isMobile mixin to use new composable and mark deprecated
• Migrate all components to use composable instead of the mixin
• The same with isFullscreen

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable

[ShGKme]

P.S. It was originally discussed in #4758

[raimund-schluessler]

@ShGKme Awesome, thanks a lot. Works well!

[susnux]

Finally got some time for testing: Works nicely 🚀

[susnux]

BTW for the tests you need to adjust the import in tests/unit/components/NcAppNavigation/NcAppNavigation.spec.js

[raimund-schluessler]

@ShGKme I rebased and adjusted the tests of NcAppNavigation. I hope, that's ok for you.

[raimund-schluessler]

@ShGKme Can we go ahead here? I didn't want to merge your PR without you agreeing to it 😉