Embed sources in CSS sourcemaps (#2668)

This flag tells Sass to embed the entire contents of the Sass files
    that contributed to the generated CSS in the source map. This may
    produce very large source maps, but it guarantees that the source
    will be available on any computer no matter how the CSS is served.

https://sass-lang.com/documentation/cli/dart-sass/#embed-sources

Author: ignisf

package.json

@@ -1,7 +1,7 @@
 {
   "scripts": {
     "build": "esbuild app/assets/javascripts/administrate/application.js --bundle --sourcemap --outdir=app/assets/builds/administrate --public-path=/assets/administrate",
-    "build:css": "sass ./app/assets/stylesheets/administrate/application.scss:./app/assets/builds/administrate/application.css ./app/assets/stylesheets/administrate-internal/docs.scss:./app/assets/builds/administrate-internal/docs.css --source-map --load-path=node_modules",
+    "build:css": "sass ./app/assets/stylesheets/administrate/application.scss:./app/assets/builds/administrate/application.css ./app/assets/stylesheets/administrate-internal/docs.scss:./app/assets/builds/administrate-internal/docs.css --source-map --embed-sources --load-path=node_modules",
     "lint:css": "stylelint app/assets/stylesheets/administrate/**/*.scss"
   },
   "dependencies": {