DeclGroup improvements, cleaner tests, and migration to `swiftlang/swift-syntax`

What's Changed

• Transition to swift-macro-testing for Macro Testing by @ajkolean in #16
• Add Wrappers for Class, Actor, Extension by @ajkolean in #17
• Migrate organisation from apple to swiftlang by @deivitaka in #18

New Contributors

• @ajkolean made their first contribution in #16
• @deivitaka made their first contribution in #18

Full Changelog: v0.4.0...v0.5.0