Demonstration of Elementary in a Gradle build

This is a fairly minimal project to get started with creating an annotation processor for Java, using Elementary 2.0 for testing.

Gradle 7 is the build tool, via Gradle Wrapper (./gradlew).
The project has been tested with Gradle 7.5.1, and it should work with Gradle 8.

Out-of-scope parts

The following is not covered:

Writing a code-generating annotation processor where the generated code requires runtime code or annotations on @Retention(RUNTIME).
You'd split such a project into two modules:
- foo-runtime, which contains the runtime artifacts that the generated code expects.
- foo-processor, which contains the annotation processor itself, and any annotations that have a @Retention other than RUNTIME.
Deployment to Maven Central (or any other repository).