update `regex!` to support multiple regexes and `expr`s

[vidhanio]

adding multiple regexes in the macro will return a tuple with all the regexes, and also allow using an expr to compile into the regex. this leaves the original api untouched, only adding more features.

[matklad]

This is an example, so I’d rather keep it simple rather than featerful.

[vidhanio]

i guess that does make sense, but in my opinion it doesn't add too much visual complexity to the documentation (although i admit the description could maybe use some work).