Add Dart PR health workflow

[mosuem]

Add workflow for PR health, checking things like breaking changes, missing exports etc.

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[robertbastian]

This adds 8 minutes of runner time to each PR, as well as 7 new jobs to the UI. As we barely touch the Dart package, I don't think this is worth the cost.

[mosuem]

Then it should be guarded with a paths filter. I would still recommend adding the workflow, as we don't want to accidentally push breaking changes, especially with tying the versioning of the Dart package to versions of other packages.

[CLAassistant]

All committers have signed the CLA.

[robertbastian]

We already have a CI job that detect breaking changes in our FFI layer, which this would just duplicate.

[Manishearth]

What are the next steps here? Perhaps @robertbastian and @mosuem need to discuss potential paths forward?

We could include this in cronjob CI, perhaps? Not sure if that makes sense for PR health stuff.