How should WebExtensions approach breaking changes?

[dotproto]

As we've discussed various topics in our public meetings and in issues on this repository, we've indirectly approached the topic of breaking changes a number of times. For the sake of helping us make progress on building consensus and aligning on a common platform, I think it would be helpful to more directly discuss the topic. Depending on how well we're able to align, we may also want to consider creating principles or guidelines to help inform how we approach making breaking changes in the future.