Creates a new command to validate that PRs don't change platform interface packages and implementations at the same time, to try to prevent ecosystem-breaking changes. See https://github.com/flutter/flutter/issues/89518 for context.
Per the explanation in the issue, this has carve-outs for:
- Changes to platform interfaces that aren't published (allowing for past uses cases such as making a substantive change to an implementation, and making minor adjustments to comments in the PI package based on those changes).
- Things that look like bulk changes (e.g., a mass change to account for a new lint rule)
Fixes https://github.com/flutter/flutter/issues/89518
