"Best-effort" conversion of full-regex rules to content-blocking format

Currently, full-regex rules are ignored when converting to content-blocking syntax because the [content-blocking specification](https://developer.apple.com/documentation/safariservices/creating_a_content_blocker)'s support for regex features is much worse than what is allowed in ABP-syntax rules.

It should still be possible to convert _some_ rules, by first verifying that the required regex featureset for each is still supported in content-blocking format. However, there will always be rules that cannot be converted.