WIP: add --select option to make checks opt-in

[jameslamb]

fixes #264

Introduces a new configuration option, --select, with usage similar to ruff's option of the same name. If this flag (or the corresponding pyproject.toml option) is supplied, it switches pydistcheck from opt-out behavior ("run all checks except those in ignore") to opt-in behavior ("run only the checks indicated by select").

For example, to say "only run the checks on file size" and avoid running any others:

pydistcheck \
  --select 'distro-too-large-compressed' \
  --max-distro-size-compressed '100M' \
  --select 'distro-too-large-uncompressed' \
  --max-distro-size-uncompressed '300M' \
  dist/*.whl