Allow to use Dart Sass instead of Node Sass

[Lyrkan]

This PR closes #422 by allowing sass (Dart Sass) as an alternative choice to node-sass.

---

The sass-loader allows to set the implementation that should be used to process Sass files. It currently accepts either sass or node-sass (default):

In theory this can already be used by setting the related option when calling Encore.enableSassLoader(), but Encore then throws an error asking you to install node-sass, even if you already have sass installed.

One way to prevent that could be to detect when options.implementation is set but in its next version the sass-loader will automatically detect which implementation is installed and use it accordingly.

So, instead of doing that, this PR allows to define alternative packages in our feature.js file, for instance:

const features = {
    sass: {
        method: 'enableSassLoader()',
        packages: [
            { name: 'sass-loader', enforce_version: true },
            [{ name: 'node-sass' }, { name: 'sass' }] // Will allow both `node-sass` and `sass`
        ],
        description: 'load Sass files'
    },
    // ...
}

If neither node-sass or sass is available, the error message will display both choices, and recommend the first one in the yarn add advice:

Note that for now using Dart Sass still requires the implementation option to be set:

Encore.enableSassLoader(options => {
    options.implementation = require('sass');
});

This shouldn't be a big issue since the default recommendation is still to use node-sass.

[weaverryan]

Wow! That's a really excellent solution. Thanks for the research and nice feature - love it!