fix(perf): change css link tag's initial rel attr to preload

Because:
 - we don't want the CSS request to block rendering

This commit:
 - change the link tag's initial rel to preload and switch to stylesheet
   on page load

Author: chenba

packages/fxa-settings/config/webpack.config.js

@@ -664,6 +664,33 @@ module.exports = function (webpackEnv) {
           filename: 'static/css/[name].[contenthash:8].css',
           chunkFilename: 'static/css/[name].[contenthash:8].chunk.css',
         }),
+      isEnvProduction &&
+        new (class {
+          apply(compiler) {
+            compiler.hooks.compilation.tap(
+              'LinkTagRewritePlugin',
+              (compilation) => {
+                HtmlWebpackPlugin.getHooks(compilation).alterAssetTagGroups.tap(
+                  'LinkTagRewritePlugin',
+                  (data) => {
+                    data.headTags.forEach((tag) => {
+                      if (
+                        tag.tagName === 'link' &&
+                        tag.attributes.rel === 'stylesheet'
+                      ) {
+                        tag.attributes.rel = 'preload';
+                        tag.attributes.as = 'style';
+                        tag.attributes.onload =
+                          "this.onload=null; this.rel='stylesheet';";
+                        tag.attributes.crossorigin = 'anonymous';
+                      }
+                    });
+                  }
+                );
+              }
+            );
+          }
+        })(),
       // Generate an asset manifest file with the following content:
       // - "files" key: Mapping of all asset filenames to their corresponding
       //   output file so that tools can pick it up without having to parse