Add linter to detect changes with ignores

To implement ACR for Altis, we need to check for any changes which introduce or change `// phpcs:ignore` lines. We also want to fail if `phpcs:disable` is detected.

This can be implemented as a new linter which is disabled by default. We could potentially use Danger.js for this to avoid implementing a whole checking engine.

Notably, this may need to operate on diffs, whereas our existing linters check against the whole codebase. This may require changes to the system.

Start by reviewing the existing linter system and how its checks run against the whole codebase. Determine whether a disabled-by-default linter can use Danger.js to inspect diffs for added or changed `// phpcs:ignore` lines and any `phpcs:disable`; done means the check reliably fails for those changes without affecting default runs.

Written by the indexing model from the issue text.