New principle: Consider many & diverse use cases to avoid overfitting

[LeaVerou]

This was brought up in our discussion of w3ctag/design-reviews#841

We should have a principle that encourages API designers to put effort in unearthing and considering as many and as diverse use cases as possible, to avoid designing their proposed solution around very specific use cases, which would make it harder or impossible to use for the broader user need.

Perhaps we should also add a requirement to the explainer that they list at least 3 separate use cases? I see a lot of reviews listing just one use case, which is a red flag for overfitting.