Build demos as subprojects

[simolus3]

This builds the three demo projects as subprojects, instead of making them individual roots that include the PowerSync SDK as a subbuild. More specifically, it:

• Updates the KMP compose version: With the old version I had issues running the example on the JVM.
• Raises the Java version we target in the :compose project to Java 11: The updated KMP compose plugin generates Java 11 bytecode that can't be inlined into some of the methods we use in :compose. We still support Java 8 in all other projects.
• Migrates the :android-supabase-todolist demo into a subproject.
• Deletes the hello-powersync demo. I don't know what purpose that demo would still have, it feels like a tiny subset of what we have in the KMP demo? I think it may be something left over from when we didn't have Desktop support in the main demo.
• Migrates the :supabase-todolist demos into subprojects.

The main benefit of using subprojects is that we can open all demos by opening the main PowerSync SDK, which is considerably easier than managing them independently. Also, we now have all our dependencies in one place and don't need to upgrade versions in different places.

I have tested the KMP demo on iOS, Android and Desktop. I've also tested the standalone Android demo on Android.

[stevensJourney]

I love the changes here.

I believe the Hello PowerSync demo was the original and somewhat basic demo we had before the Supabase todolist demo was fully functional. From my side, I don't have any concerns with removing this demo. We should update and remove and references to it in docs. E.g. https://docs.powersync.com/resources/demo-apps-example-projects#kotlin-multiplatform