ESLint config for CommonJS module syntax

Changes the eslint rules to support "require", "module.exports", and
relaxes the top-level lexical scope for each file.

FIXME: This allows a combination of CommonJS code, which becomes
unsafe global code scope if used in a "scripts" module.  What's the
correct order of migration steps?

Change-Id: I2f80732942a602c0fcff5c0dad1e2afc2640651e

.eslintrc.json

@@ -1,5 +1,8 @@
 {
 	"root": true,
+	"env": {
+		"commonjs": true
+	},
 	"extends": [
 		"wikimedia/client",
 		"wikimedia/jquery",