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

[antonok-edm]

Currently, full-regex rules are ignored when converting to content-blocking syntax because the content-blocking specification'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.

[antonok-edm]

We'd likely want to use the regex-syntax crate for this purpose - that is what's used internally in the actual regex crate.