Use build hooks to link core extension

[simolus3]

Note: I'll use the dev branch for most of the changes from the Dart SDK proposal. That way, we can merge and release everything in the end without making it more difficult to land other changes in the meantime.

Currently, linking the SQLite core extension is the responsibility of the powersync_flutter_libs package. It uses Gradle, SwiftPM and CocoaPods dependencies as well as CMake scripts to make the core extension loadable via DynamicLibrary.open from Dart. This works alright, but has two main downsides:

1. We need workarounds to link the core extension outside of Flutter apps, e.g. for tests or if users want to use the SDK in Dart CLI applications.
2. Updating the core extension is a hassle since we need to do it in multiple places.

Starting from Dart 3.10, released in November last year, we have a better option available: Build hooks! These run as part of the toolchain whenever a Dart app is built, and can contribute native code to the app. It's the responsibility of the Dart tool to consume added code in a platform-specific way. So Flutter would setup an XCFramework when building an iOS app for instance, and dart test would load the emitted library manually.

This allows us to write that bundling logic in a single place (hook/build.dart) and delete powersync_flutter_libs.

Important

Instead of deleting powersync_flutter_libs outright, I've removed all build code from that package. By then depending on the updated empty package version, we can ensure users don't end up with the new hook and the old bundling package in their Flutter app.

Tests

• powersync_core unit tests
• Test demo on Android.
• Test demo on iOS simulator.
• Test demo on iOS with testflight.
• Test demo on macOS.
• Test demo on Linux.
• Test demo on Windows.