build: switch to yarn

.gitignore

@@ -2,3 +2,4 @@ node_modules/
 lib
 testdata/generated/
 coverage/
+package-lock.json

.travis.yml

@@ -1,27 +1,26 @@
 language: node_js
 node_js: '10'
 cache:
-  directories:
-    - ~/.npm
+  yarn: true
 env:
   global:
     - FORCE_COLOR=1
 install:
-  - npm ci
+  - yarn --frozen-lockfile
 jobs:
   include:
     - stage: test
       script:
-        - npm run prettier
-        - npm run typecheck
-        - npm run tslint
-        - npm run build
-        - npm test -- --single-run --browsers ChromeHeadlessNoSandbox
+        - yarn run prettier
+        - yarn run typecheck
+        - yarn run tslint
+        - yarn run build
+        - yarn test -- --single-run --browsers ChromeHeadlessNoSandbox
         - bash <(curl -s https://codecov.io/bash)
     - stage: release
       script:
-        - npm run build
-        - npm run semantic-release
+        - yarn run build
+        - yarn run semantic-release
 stages:
   - test
   - name: release

README.md

@@ -28,12 +28,12 @@ You need to provide your own UI component (referred to as the HoverOverlay) that
 ## Development
 
 ```sh
-npm install
-npm test
+yarn
+yarn test
 
 # Helpful options:
-npm test -- --single-run      # Don't rerun on changes
-npm test -- --browsers Chrome # Only run in Chrome
+yarn test -- --single-run      # Don't rerun on changes
+yarn test -- --browsers Chrome # Only run in Chrome
 ```
 
 Development is done by running tests. [Karma](https://github.com/karma-runner/karma) is used to run
@@ -45,7 +45,7 @@ You can run specific tests by [adding `.only` to `describe` or `it` calls](https
 ## Releases
 
 Releases are done automatically in CI when commits are merged into master by analyzing [Conventional Commit Messages](https://conventionalcommits.org/).
-After running `npm install`, commit messages will be linted automatically when committing.
+After running `yarn`, commit messages will be linted automatically when committing.
 You may have to rebase a branch before merging to ensure it has a proper commit history.
 
 ## Glossary