feat: CI check for validating changedFilePatterns

[juliusmarminge]

In the latest version of changeset, @Andarist added the changedFilePatterns option which let's you configure when a changeset should be needed and not.

It would be nice for the bot to validate that this config is adhered to. For example, given the following config:

{
  "...": "",
  "ignore": ["www"],
  "changedFilePatterns": ["src/**"]
}

The bot would trigger a CI step to check the changed file for the PR and validate it accordingly:

• If a PR makes changes to src/**, and there is a changeset - CI should pass.
  • Note that we need to check the ignore field here, i.e. changes to www/src/** should not require changeset.
• If a PR makes changes to src/**, and there isn't a changeset - CI should fail.
• If a PR doesn't make changes to src/**, yet there is a changeset - CI should fail.

@Andarist suggested you could do this manually using changeset status but would be nice to have this supported by the bot out of the box - perhaps there's some other nice integration with the current comment the bot leaves that could be made here too in regard to this config option / CI step 🤔