Prepare stable 2.0.0 release

[simolus3]

This:

• Upgrades to the stable versions of sqlite_async and drift_sqlite_async.
• Re-generates CMake definitions in demos.
• Adds a test verifying that our libraryVersion constant always matches the version in pubspec.yaml (making it harder to forget that step when preparing a release).
• Prepares a stable release of package:powersync version 2.0.0 and package:powersync_attachments_helper version 0.7.0.