ci/quality checks

[caverac]

Introduces a Gradle-based build system, code quality tooling, and a GitHub Actions CI pipeline to the project

Changes can be organized in two buckets

Code quality

These tools are introduced to the project

1. Linting: Checkstyle 10.21
2. Formatting: Spotless
3. Testing: JUnit 5

And configured to be lenient, over time they can tightened to ensure code quality in the project, and to catch potential regressions when new features are introduced.

Automation

1. Github workflows
2. Pre-commit hooks

These run automatic tests on code quality on PRs

How to use

Command	Purpose
./gradlew compileJava	compile
./gradlew test	run tests
./gradlew jacocoTestReport	generate coverage report
./gradlew checkstyleMain	run checkstyle
./gradlew spotbugsMain	run spotbugs
./gradlew spotlessCheck	check formatting
./gradlew spotlessApply	auto-fix formatting

[caverac]

https://github.com/CategoricalData/CQL/actions/runs/23004104156

This is a chicken-and-egg problem: the CI workflow itself is part of this PR, so it can't run until it's merged.