Add validation loosening awareness to all checks

Some schema property checks only take into consideration validation tightening as a breaking change, but this isn't always the case.

Depending on how the API is used, there may be cases where you want to be:
- Strict on tightening of validations but not loosening
- Strict on loosening of validations but not tightening
- Strict on both loosening and tightening of validations

Start by locating the schema property checks that currently detect validation tightening and trace how their breaking-change results are combined. Determine how the three requested strictness modes should be represented and applied to every check, then verify that tightening-only, loosening-only, and both-direction behavior are covered.

Written by the indexing model from the issue text.