fix - ts typings now export slugify correctly

[neonerd]

This fixes the problem with Webpack + TS-Loader not working correctly after slugify added TS definitions.

Fixes #16

[simov]

Thank you! 👍

[sylvaindumont]

Thanks for the fix, I chose commonjs export instead of es6 export because it was used in others browser/node modules exporting a single function (ex: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/haversine/index.d.ts).
es6 export worked better for me but I followed what seemed to be a convention to avoid breaking builds, but it did the opposite.
Your es6 export works well when compiling with commonjs modules and es6 modules, thank you again.

[karlhorky]

This change breaks the new "module": "node16" setting in tsconfig.json. I'll create a reproduction and PR reverting this change. I think @sylvaindumont's original approach was actually correct, because slugify is indeed a CommonJS module.

Hopefully bundler module resolution with webpack + ts-loader (and other bundlers) have since adapted and now also can also deal with the CommonJS-style export, because it seems to be the right way to do this.

[karlhorky]

Reproduction of problem + PR to revert to CommonJS export: #171