Lazily require node-sass

[HoldYourWaffle]

Closes #747.

Implementation of #747 for your convenience.
I'm technically in violation of the contributor guidelines by opening this PR without prior approval, but I need this functionality right now and I want to use my fork in the meantime.

I'm not sure how I should update the changelog, should I add a new version "heading"?

[ncheikh]

Our company also would like to see this merged as it should allow for node 12 support with the node-sass dependency.

[harshjv]

@dlmanning can you please check & merge this PR?

[3cp]

Should not this be auto-detection?

let compiler = gulpSass.compiler;
if (!compiler) {
  try {
    compiler = require('node-sass');
  } catch (e) {
    try {
      compiler = require('sass');
    } catch (e) {
      return cb(new PluginError(
        PLUGIN_NAME,
        "No compiler found!\nPlease install one of the npm packages: node-sass or sass, or set compiler in code:\n    const sass = require('gulp-sass');\n    sass.compiler = require('sass');\n"
      ));
    }
  }
}

So that user don't need to set dart-sass by hand. For reference, webpack's sass-loader does this kind of auto-detection.

[HoldYourWaffle]

That makes a lot of sense, however I'm not in a position to fix this at the moment.
I'll probably get to it somewhere next week (if nobody else has yet).

[MartijnCuppens]

Thoughts on changing this to:

You can either choose node-sass or sass:

npm install gulp-sass sass --save-dev

or:

npm install gulp-sass node-sass --save-dev

This way, users can copy the one-liner they prefer?

[HoldYourWaffle]

I think that makes sense. Perhaps we should also add some additional information on the differences?

[hasghari]

Can I help with anything to speed this along? Without this change, we are blocked on Node 14.

[b1rdex]

I've just found https://www.npmjs.com/package/gulp-dart-sass that can be used as a drop-in replacement.

[xzyfer]

Thanks all, close this to focus efforts on #802 which is more rounded at this point in time.