Switch to Dart Sass for contributing site

Dart Sass is the primary implementation of Sass and is distributed as
the pure JavaScript `sass` package on npm. This configures gulp-sass to
use Dart Sass, instead of Node Sass.

https://github.com/dlmanning/gulp-sass#basic-usage

Author: tysongach

Gulpfile.js

@@ -3,6 +3,8 @@ const browserSync = require('browser-sync').create();
 const gulp = require('gulp');
 const sass = require('gulp-sass');
 
+sass.compiler = require('sass');
+
 const paths = {
   markup: {
     src: './contrib/**/*.html',

package-lock.json

@@ -29,6 +29,7 @@
     "eslint": "5.16.0",
     "gulp": "^4.0.2",
     "gulp-sass": "^4.0.2",
+    "sass": "^1.23.0",
     "stylelint": "^10.1.0"
   },
   "scripts": {