feat: Add regex feature flag

[LucasPickering]

First off, love this library, thanks for it. For my use case I don't really need the regex functions, and it's a really large dependency to pull in (adds ~8s to my build process).

This change hides the regex functions (match and search) behind a feature flag regex, which is enabled by default. This allows consumers to completely eliminate regex as a dependency if they choose. Also added some docs on feature flags.

I wasn't sure if I should update the changelog since it looks like it might be auto-generated from git cliff, let me know what you'd like.

[hiltontj]

@LucasPickering - thanks for the PR, this is a great idea! I am on vacation this week so when I am back next week I will take a closer look and can cut this into a release.

I think this might need to be a 0.7.0 release, since anyone using default-features = false to disable the "functions" feature would potentially break things.

As for the changelog, I have been updating that manually. You are welcome to update it on this PR if you want or I can do it in a follow-on PR.

[LucasPickering]

@hiltontj I agree about the breaking change, it would be an easy upgrade but still deserves 0.7. I'll add to the changelog. Incidentally I realized I no longer need this flag because I ended up pulling regex into my project as part of an unrelated dependency, but this might still be helpful for others 😛

[hiltontj]

LGTM - Thank you for updating the changelog as well!

[hiltontj]

Oh shoot, the docs link to tracing in the feature flags section is broken -perhaps because tracing is behind a non-default feature flag? Either way, you could probably just link to the latest tracing docs directly: https://docs.rs/tracing/latest/tracing/