feat: configure Babel for browsers with ES module support

Author: colinrotherham

.browserslistrc

@@ -1,3 +1,7 @@
+[javascripts]
+supports es6-module
+
+[stylesheets]
 > 0.1% in GB and not dead
 last 6 Chrome versions
 last 6 Firefox versions

babel.config.js

@@ -1,4 +1,5 @@
 module.exports = {
+  browserslistEnv: 'javascripts',
   presets: [
     [
       '@babel/preset-env',

gulp/tasks/styles.js

@@ -36,7 +36,11 @@ function compileStyles(assetPath, { srcPath, destPath, output = {} }) {
     }
 
     // Apply PostCSS transforms (e.g. vendor prefixes)
-    const processor = postcss([autoprefixer(), cssnano()])
+    const processor = postcss([
+      autoprefixer({ env: 'stylesheets' }),
+      cssnano({ env: 'stylesheets' })
+    ])
+
     const result = await processor.process(css, {
       from,
       to,