Move to yarn and renovate like other ampproject repos (#27)

* Move to yarn and renovate like other ampproject repos
* Migrate to yarn
* Add pr driven tests
* Update depedencies
* Reduce testing to Chrome only for now
* Try ChromeHeadless
* Use ChromeHeadless for Github Actions

Author: kristoferbaxter

.github/workflows/karma.yml

@@ -0,0 +1,18 @@
+name: Karma
+
+on: [pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [12.x]
+    steps:
+    - uses: actions/checkout@v1
+    - name: Use Node.js ${{ matrix.node-version }}
+      uses: actions/setup-node@v1
+      with:
+        node-version: ${{ matrix.node-version }}
+    - run: yarn install
+    - run: yarn test-action

.gitignore

@@ -1,3 +1,4 @@
 node_modules/
 dist/
-build/
+build/
+package-lock.json

README.md

@@ -24,21 +24,21 @@ dimensions of elements.
 ### Build
 
 ```shell
-npm run build
-npm run build-watch
+yarn build
+yarn build-watch
 ```
 
 ### Test
 
 ```shell
-npm run test
-npm run test-watch
+yarn test
+yarn test-watch
 ```
 
 ### Demos
 
 To build, serve, and open a browser tab with the demos:
 
 ```shell
-npm run demo
+yarn demo
 ```