Add Sass build config

This was done automatically using `./bin/rails css:install:sass`

I've added `--quiet-deps` here, as it suppresses deprecation warnings
for the GOV UK frontend library, as is recommended here:

alphagov/govuk-frontend#2238

Procfile.dev

@@ -1,3 +1,4 @@
 worker: bundle exec sidekiq -C config/sidekiq.yml
 web: bin/rails server -p 3000
 js: yarn build --watch
+css: yarn build:css --watch

app/assets/config/manifest.js

@@ -1,3 +1,2 @@
 //= link_tree ../images
-//= link_directory ../stylesheets .css
 //= link_tree ../builds

app/assets/stylesheets/application.sass.scss

@@ -0,0 +1 @@
+// Entry point for your Sass build

package.json

@@ -8,6 +8,7 @@
     "accessible-autocomplete": "^2.0.2",
     "govuk-frontend": "^3.11.0",
     "rollup": "^3.7.5",
+    "sass": "^1.57.0",
     "webpack": "^4.46.0",
     "webpack-cli": "^3.3.12"
   },
@@ -20,6 +21,7 @@
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
     "build": "rollup -c --bundleConfigAsCjs rollup.config.js",
+    "build:css": "sass ./app/assets/stylesheets/application.sass.scss:./app/assets/builds/application.css --no-source-map --load-path=node_modules --quiet-deps"
   },
   "repository": {
     "type": "git",

yarn.lock

@@ -3469,6 +3469,11 @@ iferr@^0.1.5:
   resolved "https://registry.yarnpkg.com/iferr/-/iferr-0.1.5.tgz#c60eed69e6d8fdb6b3104a1fcbca1c192dc5b501"
   integrity sha1-xg7taebY/bazEEofy8ocGS3FtQE=
 
+immutable@^4.0.0:
+  version "4.1.0"
+  resolved "https://registry.yarnpkg.com/immutable/-/immutable-4.1.0.tgz#f795787f0db780183307b9eb2091fcac1f6fafef"
+  integrity sha512-oNkuqVTA8jqG1Q6c+UglTOD1xhC1BtjKI7XkCXRkZHrN5m18/XsnUp8Q89GkQO/z+0WjonSvl0FLhDYftp46nQ==
+
 import-cwd@^2.0.0:
   version "2.1.0"
   resolved "https://registry.yarnpkg.com/import-cwd/-/import-cwd-2.1.0.tgz#aa6cf36e722761285cb371ec6519f53e2435b0a9"
@@ -5935,6 +5940,15 @@ sass@^1.38.0:
   dependencies:
     chokidar ">=3.0.0 <4.0.0"
 
+sass@^1.57.0:
+  version "1.57.0"
+  resolved "https://registry.yarnpkg.com/sass/-/sass-1.57.0.tgz#64c4144ed4e1c0ccb96dc18aef2c424cdbc0c12b"
+  integrity sha512-IZNEJDTK1cF5B1cGA593TPAV/1S0ysUDxq9XHjX/+SMy0QfUny+nfUsq5ZP7wWSl4eEf7wDJcEZ8ABYFmh3m/w==
+  dependencies:
+    chokidar ">=3.0.0 <4.0.0"
+    immutable "^4.0.0"
+    source-map-js ">=0.6.2 <2.0.0"
+
 sax@~1.2.4:
   version "1.2.4"
   resolved "https://registry.yarnpkg.com/sax/-/sax-1.2.4.tgz#2816234e2378bddc4e5354fab5caa895df7100d9"
@@ -6201,6 +6215,11 @@ source-list-map@^2.0.0:
   resolved "https://registry.yarnpkg.com/source-list-map/-/source-list-map-2.0.1.tgz#3993bd873bfc48479cca9ea3a547835c7c154b34"
   integrity sha512-qnQ7gVMxGNxsiL4lEuJwe/To8UnK7fAnmbGEEH8RpLouuKbeEm0lhbQVFIrNSuB+G7tVrAlVsZgETT5nljf+Iw==
 
+"source-map-js@>=0.6.2 <2.0.0":
+  version "1.0.2"
+  resolved "https://registry.yarnpkg.com/source-map-js/-/source-map-js-1.0.2.tgz#adbc361d9c62df380125e7f161f71c826f1e490c"
+  integrity sha512-R0XvVJ9WusLiqTCEiGCmICCMplcCkIwwR11mOSD9CR5u+IXYdiseeEuXCVAjS54zqwkLcPNnmU4OeJ6tUrWhDw==
+
 source-map-resolve@^0.5.0:
   version "0.5.3"
   resolved "https://registry.yarnpkg.com/source-map-resolve/-/source-map-resolve-0.5.3.tgz#190866bece7553e1f8f267a2ee82c606b5509a1a"