Improve unicode support

[Richienb]

Require Node.js 16 and move to ESM

Intl.Segmenter is much more accurate than the regex was.

Fixes #50, Fixes #33

[sallar]

Hi.

Thanks. Im happy to discuss merging this in, I have two concerns though:

• Moving to ESM completely might break some projects that don't support it yet.
• Support for Intl.Segmenter is not universal.

Maybe we can release the current (non-esm) version to npm using a @legacy tag or something and then do a breaking release with the new changes. What do you think?

[Richienb]

I'm ok with breaking ESM support but a lack of Intl.Segmenter support is a problem. Not sure what to do.

[sallar]

@Richienb then I think I can add a legacy sub-import to fix this. eg:

if imported from ES6 -> Intl.Segmenter
if imported from commonJS -> older version

OR

import stringz from 'stringz' -> Intl.Segmenter
import stringz from 'stringz/legacy' -> older version