Regex for hex string should be somewhere easily accessible in aztec.js

[sklppy88]

          The `i` flag makes it case insensitive, so lowercase `a-f` is included. I actually don't see anything anywhere that is in aztec.js, which means maybe we should expose some of this to use elsewhere ? I'll make an issue for this to be included in another pr.

Originally posted by @sklppy88 in #7366 (comment)