Split Filter and Builder into separate crate

[vadixidav]

I am finding the Filter to be useful for creating my own logger in a custom wasm environment. This functionality is useful, but to pull it in I also have to pull in the whole env_logger crate. The readme for this crate reads that it should only be used for binaries, which makes sense, but this one component of it is useful for making libraries. Is it possible that the Filter and Builder can be split into their own crate? It seems like a good thing to refactor out. Essentially, the whole filter module.

[jplatte]

I think this has been brought up somewhere before. CC @KodrAus who knows a lot more about the overall log ecosystem than me.