Add feature to use regex-lite instead?

[yujincheng08]

Hi, I am using actix-web for my project and it works like a charm. However, I have a tiny concern about the binary size. I used cargo bloat and figured out that regex contributes a large part to the binary size, which is used by actix-web. I found out that rust-lang/regex offers an alternative to regex with smaller binary size but with slower performance and limited Unicode support. I wonder if it's possible to add a feature to use regex-lite for users with binary size concerns? I tried to migrate to regex-lite and it's quite easy. If it's possible, I am more than happy to open a PR.

[robjtede]

I'd be happy to see a PR for this change, though we should be careful and consider Rust best practices when it comes to feature flags being additive.

Edit: PR from @yujincheng08 #3291