Add --set-exit-if-changed (or similar) option to pub downgrade

[jakemac53]

Use case

You want to validate your minimum constraints are valid across all your package dependencies on CI.

Today, you can run a pub downgrade but it does not guarantee that you get the actual minimum versions specified there.

You can also pass --tighten which will update the min constraints to whatever could actually be solved for (and thus tested).

However, these both always give a zero exit code as long as there was a valid version solve. If you properly want to validate your package (and its constraints), you want to run something on CI which will give a non-zero exit code if the constraints need to be updated to reflect what can actually be tested.

The dart format command has a --set-exit-if-changed flag which is commonly used for this task, and I propose something similar here.