Tracking: Test generator MVP

Test generator MVP to use after `configlet generate` for practice exercises:

- [x] #638 Integrate [POC test generator](https://github.com/tomasnorre/exercism-tests-generation) into new directory `contrib/generator`, so it is not installed in CI
- [x] #638 Add a parameter to choose the exercise by slug
- [x] #638 Use local `configlet` cached problem specification repository as information source
- [ ] #663 Do not do anything, if exercise has no canonical-data (there are some)
- [ ] #663 Use `/exercises/practice/${slug}/.meta/config.json>files>test` and check that there is only one entry
- [x] #638 Write tests to already created `/exercises/practice/${slug}`
- [x] #638 Have test class skeleton
- [ ] #663 Have test class DocBlock with "PHP track rules"
- [x] #638 Have testdox with description
- [x] #638 Have UUID in DocBlock
- [x] #638 Have all tests marked incomplete
- [ ] #663 Have `$input`, `$expected`
- [ ] #663 Have `$subject = new ${PascalSlug};`, `$actual = $subject->${property}($input);` and `assertSame($expected, $actual);`
- [ ] #663 Handle unknown data, so the user can act on it
- [ ] #663 Handle nested groups of tests
- [ ] Bonus: Add composer script to handle exercise generation from scratch. The problem is handling the `slug` required for exercise generation. That requires some additional scripting, so that only those commands get the slug that need it and the others do not fail.
- [ ] #663 adds unit tests, but no integration tests ~~Bonus: Add golden tests for test generation of some exercises~~
- [ ] #663 handles unknow data instead ~~Bonus: Add JSON validation using the `configlet` scheme (if that exists), else at least field existance and character sets~~
- [ ] Bonus: Add PHP CodeSniffer (with rules for test-generator only) and more static analysis

**Not MVP**:

- Producing production ready tests
- [scenarios](https://github.com/exercism/problem-specifications?tab=readme-ov-file#scenarios)
- [Reimplementation](https://github.com/exercism/problem-specifications?tab=readme-ov-file#changing-tests)
- Ignoring tests marked with `include = false` in `tests.toml`

Open questions:

- Should it be a single command app or do we want to add more commands like `update-exercise` (or so)?
- Should we use another code producer library like [`laminas/laminas-code`](https://docs.laminas.dev/laminas-code/generator/intro/)?