Eliminate regex and once_cell dependencies.

[nicoburns]

Objective

Reduce compile time / binary size impact of Comrak for projects that are not otherwise using the regex crate.

Changes made

• Replace use of regex in a doctest with a simple String::contains
• Replace the usage of regex in anchorization code with unicode_categories crate which comrak already depends on.
• Removes once_cell dependendency, as it was only being used to cache a Regex

Comrak also currently also pulls in regex as a build dependency of caseless. But I have also sent that crate a PR to remove it's usage.

[kivikakk]

Wonderful. Thank you!