Configure Dummy app to serve ActionText assets

Without adding the [`sassc-rails` gem][sassc-rails] as a dependency,
attempts include the `trix.js`-provided stylesheets into the Asset
Pipeline [result in an error][sassc-rails-error].

Additionally, this commit extends the `.github/workflows/tests.yml` to
[include NodeJS support][setup-node], and to install the Dummy
application's Yarn dependencies during the setup phase (through the
[`yarn:install` Rails task][yarn:install], and to [cache the installed
Yarn packages][cache].

Finally, declare `test/dummy/config/initializers/assets.rb` to [configure
the Asset Pipeline's search path][asset-initializer] to include the
_project root's_ `node_modules/` directory.

[sassc-rails]: https://github.com/sass/sassc-rails
[sassc-rails-error]: sass/sassc-rails#114
[setup-node]: https://github.com/actions/setup-node/tree/v1#usage
[yarn:install]: https://github.com/rails/webpacker/blob/v5.0.1/lib/tasks/webpacker/yarn_install.rake
[cache]: https://github.com/actions/cache/blob/v1/examples.md#node---yarn
[asset-initializer]: https://guides.rubyonrails.org/asset_pipeline.html#search-paths

.github/workflows/tests.yml

@@ -24,22 +24,35 @@ jobs:
       with:
         ruby-version: "${{ matrix.ruby }}"
 
+    - name: "Install NodeJS 12.x"
+      uses: "actions/setup-node@v1"
+      with:
+        node-version: "12.x"
+
     - name: "Generate lockfile"
       run: |
         bundle config path vendor/bundle
         bundle lock
 
-    - uses: "actions/cache@v1"
+    - name: "Cache installed Ruby dependencies"
+      uses: "actions/cache@v1"
       with:
         path: "vendor/bundle"
         key: bundle-${{ hashFiles('Gemfile.lock') }}
 
+    - name: "Cache installed Yarn dependencies"
+      uses: "actions/cache@v1"
+      with:
+        path: "node_modules"
+        key: yarn-${{ hashFiles('yarn.lock') }}
+
     - name: "Install Dependencies"
       run: |
         sudo apt-get update
         sudo apt-get -yqq install libsqlite3-dev chromium-browser
 
         bundle install --jobs 4 --retry 3
+        bin/rails app:yarn:install
 
         bin/rails app:webdrivers:chromedriver:update
 

Gemfile

@@ -22,5 +22,6 @@ else
   rails_constraint = "~> #{rails_version}.0"
 end
 
+gem "sassc-rails"
 gem "rails", rails_constraint
 gem "webpacker"

test/dummy/config/initializers/assets.rb

@@ -0,0 +1,4 @@
+# Add additional assets to the asset load path.
+# Rails.application.config.assets.paths << Emoji.images_path
+# Add Yarn node_modules folder to the asset load path.
+Rails.application.config.assets.paths << Rails.root.join("../../node_modules")