New principle: only extend problematic and non-consensus features if the extension acts to fix the problems

[jyasskin]

This was brought up in our discussion of Protected Audiences Bidding and Auction Services API.

This might fit into https://w3ctag.github.io/design-principles/#leave-the-web-better.

The idea is that improving a problematic feature will tend to increase use of that feature, which increases any problems caused by use of the feature, so the improvements should compensate for the increase in use with a decrease in the problems-per-use.

Extensions shouldn't have to completely fix the problems in order to be a good idea, just have some indication that the total amount of problems will go down.