Fix doc tests

[greyblake]

Steps to reproduce

With Rust 1.80.1

• cd /nutype (the dir with nutype crate)
• cargo test
• See number of failing test, e.g.

---- nutype/src/lib.rs - (line 123) stdout ----
error: To validate string types with regex, the feature `regex` of the crate `nutype` must be enabled.
       IMPORTANT: Make sure that your crate EXPLICITLY depends on `regex` and `lazy_static` crates.
       And... don't forget to take care of yourself and your beloved ones. That is even more important.
 --> nutype/src/lib.rs:129:19
  |
9 | #[nutype(validate(regex = "^[0-9]{3}-[0-9]{3}$"))]
  |                   ^^^^^

To do

• Found out when the regression was introduced
• Fix
• Setup CI to catch this in the future
• Update Justfile to run doc tests in nutype crate

[greyblake]

Commit that introduced the the problem: 367f638