TypeScript: use union types instead of enums

[slorber]

I'm using latest webpack-merge 5.*

There are a ton of reasons to not use enums in TypeScript anymore (it's not std JS), particularly if you don't even benefit from its nominal typing.

The main one I'd give in this context is that your readme examples do not work out of the box, as it requires to import the enum before being able to use it.

[bebraw]

Can you check #185 ? To avoid breaking backwards-compatibility, I added strings as an option there so that IDEs shouldn't complain anymore.

[slorber]

thanks @bebraw , LGTM!