perf(infrastructure): Cut build time in half with opt-in env var (#1128)

A new `GENERATE_SOURCE_MAPS=false` env var disables source map generation in dev mode, which cuts build and recompile times in half.

Invoked like so:

```bash
GENERATE_SOURCE_MAPS=false npm run dev
```

webpack.config.js

@@ -26,6 +26,10 @@ const OUT_PATH = path.resolve('./build');
 const PUBLIC_PATH = '/assets/';
 const IS_DEV = process.env.MDC_ENV === 'development';
 const IS_PROD = process.env.MDC_ENV === 'production';
+const GENERATE_SOURCE_MAPS =
+    process.env.MDC_GENERATE_SOURCE_MAPS === 'true' ||
+    (process.env.MDC_GENERATE_SOURCE_MAPS !== 'false' && IS_DEV);
+const DEVTOOL = GENERATE_SOURCE_MAPS ? 'source-map' : false;
 
 const banner = [
   '/*!',
@@ -50,20 +54,20 @@ const CSS_LOADER_CONFIG = [
   {
     loader: 'css-loader',
     options: {
-      sourceMap: true,
+      sourceMap: GENERATE_SOURCE_MAPS,
     },
   },
   {
     loader: 'postcss-loader',
     options: {
-      sourceMap: IS_DEV,
-      plugins: () =>[require('autoprefixer')({grid: false})],
+      sourceMap: GENERATE_SOURCE_MAPS,
+      plugins: () => [require('autoprefixer')({grid: false})],
     },
   },
   {
     loader: 'sass-loader',
     options: {
-      sourceMap: true,
+      sourceMap: GENERATE_SOURCE_MAPS,
       includePaths: glob.sync('packages/*/node_modules').map((d) => path.join(__dirname, d)),
     },
   },
@@ -106,7 +110,7 @@ module.exports = [{
   devServer: {
     disableHostCheck: true,
   },
-  devtool: IS_DEV ? 'source-map' : false,
+  devtool: DEVTOOL,
   module: {
     rules: [{
       test: /\.js$/,
@@ -133,7 +137,7 @@ module.exports = [{
   devServer: {
     disableHostCheck: true,
   },
-  devtool: IS_DEV ? 'source-map' : false,
+  devtool: DEVTOOL,
   module: {
     rules: [{
       test: /\.js$/,
@@ -190,7 +194,7 @@ module.exports = [{
   devServer: {
     disableHostCheck: true,
   },
-  devtool: IS_DEV ? 'source-map' : false,
+  devtool: DEVTOOL,
   module: {
     rules: [{
       test: /\.scss$/,
@@ -220,7 +224,7 @@ if (IS_DEV) {
     devServer: {
       disableHostCheck: true,
     },
-    devtool: 'source-map',
+    devtool: DEVTOOL,
     module: {
       rules: [{
         test: /\.scss$/,