[RFC] Use rollup to build the code and generate CommonJS and UMD version

[realityking]

This is probably quite a bit to swallow, so quick overview what this PR does:

• It replaces most of what's happening in build.js with rollup.
• It builds slugify in 2 versions: UMD (same as before), CommonJS
• It changes package.json to specific the CommonJS file as main

The benefit of all this trouble are bundle size and easier static analysis for IDEs and other tools. The UMD version is 40 bytes smaller when minified with UglifyJS (~1%), the new CommonJS version is even 238 bytes smaller (~4.2%). Static analysis for IDEs and tools like rollup becomes easier because they generally can't figure out what's exported by an UMD module.

It's also a good step towards ESM support but that will require a bit more work.

What do you think?

[realityking]

I've been playing grave digger and updated this PR to account for the refactoring since. They actually made this significantly easier as the new structure is much better suited for rollup.

Please let me know what you think.

[simov]

Thanks @realityking, I'll have a look at it.

[simov]

@realityking your PR is looking great, thanks for your work on it. However, I don't like the fact that building the module is now mandatory, besides injecting the charmap. Lets keep it open for now, keeping it up to date will be very easy.

[realityking]

I pushed this a little further by also adding an ESM build.

[realityking]

Rebased this (ancient) PR onto current master. The only other change is adding a tmp folder to avoid a 2nd copy of locales.json to be in the repo.

The rollup version is ancient as it's the last one to support Node.js 8. CI would have to move to 14+ or better 18+ for newer versions.

Happy to flesh this out into full ESM support if you're ok with the approach @simov @stscoundrel