Adopt & enforce the mozilla-central Prettier config (mozilla#467)

Taken from:
https://searchfox.org/mozilla-central/rev/49011d374b626d5f0e7dc751a8a57365878e65f1/.prettierrc

Explicitly include `arrowParens: avoid` to keep the Prettier v1 default.

.github/workflows/nodejs.yml

@@ -31,5 +31,6 @@ jobs:
       - uses: actions/setup-node@v3
       - run: npm install --legacy-peer-deps
       - run: npm run lint
+      - run: npx prettier --check .
       - run: npm run coverage
       - run: npx nyc report --reporter=text

.prettierignore

@@ -0,0 +1,4 @@
+.nyc_output/
+bench/
+coverage/
+node_modules/

.prettierrc

@@ -0,0 +1,7 @@
+{
+  "arrowParens": "avoid",
+  "endOfLine": "lf",
+  "printWidth": 80,
+  "tabWidth": 2,
+  "trailingComma": "es5"
+}

package.json

@@ -74,7 +74,7 @@
     "dev:watch": "watch 'npm run coverage' lib/ test/",
     "predev": "npm run setup",
     "dev": "npm-run-all -p --silent dev:*",
-    "prettier": "prettier --write *.* lib/ test/",
+    "prettier": "prettier --write .",
     "clean": "rm -rf coverage .nyc_output",
     "toc": "doctoc --title '## Table of Contents' README.md && doctoc --title '## Table of Contents' CONTRIBUTING.md"
   },