Streamline contribution commands #1066

This is a plan to stramline our contribution/ commands.

1. Create a structured command directory (♻️ Move checkDeprecatedExercises.php as symfony command #1067)
2. Move checkDeprecatedExercises.php as a command (♻️ Move checkDeprecatedExercises.php as symfony command #1067)
3. Move contribution/generator as a command (currently a fully fledged symfony application) (♻️ Move contribution/generator as symfony command #1068)
4. Move exercises/concept/bootstrap.sh as a command (or delete it if it is a duplicate of 3.) (♻️ Move contribution/generator as symfony command #1068)
5. Make a test generator from templates (✨ Test Generator #674)
6. Make contribution commands more discoverable (documentation in README)

Going further:

1. test generator: Run in CI to check files outputs are consistent with template
2. test generator: Run in CI on a monthly basis to re-generate the tests (capturing their upstream updates)
3. test generator: Write more tests
4. test generator: convert more exercises
5. exercise generator: make it generate template file
6. automatically fetch configlet
7. kickstart documentation, ideally a ./main.php exercism:init <slug> would give enough information and generate enough files
8. maintain documentation, ideally a ./main.php exercism:sync would give enough information and generate enough files (kind of linked to point 2)