Add example

[kangalio]

Fixes #22

Should the example doctest adhere to the MSRV too? If so, I will replace the let-else.

Currently, README.md and the crate root docs are duplicated. Should I fix this via #[doc = include_str!("README.md")] in a separate PR? Gated behind docsrs cfg to comply with MSRV policy.

[mibmo]

Could also use is_some_and to simplify, e.g.

pub fn is_identifier(s: &str) -> bool {
   let mut chars = s.chars();
   let first_valid = chars.next().is_some_and(is_xid_start);
   let rest_valid = chars.all(is_xid_continue);
   first_valid && rest_valid
}

Bit verbose, but is also just a matter of preference of course. :)

[mibmo]

Regardless just having an example would be much better than the status quo.