[Android] Expose a Composable rich text editor

[bmarty]

To integrate the editor to an Android application written using Jetpack Compose, the Android library should expose a Composable editor, i.e. a method annotated with @Composable.

Will be useful for element-hq/element-x-android#1, however this is not blocking us.

Tasks

• [Android] Add skeleton for Compose support #754
  • Theming and dark mode
• [Android] Add formatting actions to compose library #755
• [Android] Expose style configuration for compose #758
• [Android] Publish compose library artifact #760
  • Add usage documentation
• Fix: [Android] Fix publication task for compose library #764
• [Android] Expose more compose options #766
  • Focus API
  • Line count API
  • Text and cursor style settings
• [Android] Add preview support for compose #767
• Release

Out of scope

• Feature parity with View library