This change adds the flag `--check-extras` to `tidy`. It accepts a comma
separated list of any of the options:

- py (test everything applicable for python files)
- py:lint (lint python files using `ruff`)
- py:fmt (check formatting for python files using `black`)
- shell or shell:lint (lint shell files using `shellcheck`)

Specific files to check can also be specified via positional args.
Examples:

- `./x test tidy --check-extras=shell,py`
- `./x test tidy --check-extras=py:fmt -- src/bootstrap/bootstrap.py`
- `./x test tidy --check-extras=shell -- src/ci/*.sh`
- Python formatting can be applied with bless:
  `./x test tidy --ckeck-extras=py:fmt --bless`

`ruff` and `black` need to be installed via pip; this tool manages these
within a virtual environment at `build/venv`. `shellcheck` needs to be
installed on the system already.