Add Webpack 4 Support

[acao]

This is the only change that is required to support Webpack 4, works great for me

I see peerDependencies for this library goes back to webpack 2, so I wanted to help make that a thing!

I wish the conditional could be evaluated only once for optimal perf, but it needed to be scoped for this. Perhaps I could set a flag the first time the conditional is evaluated:

let webpack5 = true;

function makeLoader() {
    return function (source, inputSourceMap) {
     // cheaper boolean conditional. 
     // if webpack5 is false the first time, don't check again
      if(webpack5 && 'getOptions' in this) {
         webpack5 = false
      }
      let loaderOptions = webpack5 ? this.getOptions() : require('loader-utils').getOptions(this)
    }
}

Do you have a way of benchmarking the webpack loader? Might be a negligible difference but worth considering

Note: Oddly, it looks like github.dev applied some formatting as well when I created this using this new approach, thought I'd try it

[CLAassistant]

All committers have signed the CLA.

[kdy1]

Can you sign the CLA?

[acao]

@kdy1 thank you for this awesome project!