Feature: Unicode support

[conartist6]

Currently I'm using parsing expressions with regexpp which actually has most of the expensive parts of unicode support (the definitions of all the unicode character classes) baked into it. It should be fairly easy to expose that functionality in the current setup, though I would also enjoy having a lighter weight version of the package that did not offer unicode functionality (but also would not incur unicode's hit to bundle size).

[conartist6]

Prior art:

• https://www.npmjs.com/package/char-regex
• https://www.npmjs.com/package/string-length