Add import extension rule

Signed-off-by: Christopher Ng <chrng8@gmail.com>

index.js

@@ -111,5 +111,10 @@ module.exports = {
 		}],
 		// require object literal shorthand syntax
 		'object-shorthand': ['error', 'always'],
+		// Warn when file extensions are not used in import paths
+		'import/extensions': ['warn', 'ignorePackages', {
+			// Require file extension for json imports
+			json: 'always',
+		}],
 	},
 }