Improve ESLint setup and fix errors (#1656)

* Improve ESLint setup

- Uses `eslint:recommended` + a handful more rules.
- Uses .eslintignore so that editors can understand which files to lint.
- Uses .eslintrc<strong>.js</strong> so more editors get syntax highlighting.

* Fix ESLint errors

About half of them were fixed by `eslint . --fix`.

bin/prettier.js

@@ -129,6 +129,7 @@ function getTrailingComma() {
         "Warning: `--trailing-comma` was used without an argument. This is deprecated. " +
           'Specify "none", "es5", or "all".'
       );
+      return "es5";
     case "es5":
       return "es5";
     case "all":