chore: add strict mode (#2974)

src/errors/DJSError.js

@@ -1,3 +1,5 @@
+'use strict';
+
 // Heavily inspired by node's `internal/errors` module
 
 const kCode = Symbol('code');

src/errors/Messages.js

@@ -1,3 +1,5 @@
+'use strict';
+
 const { register } = require('./DJSError');
 
 const Messages = {

src/errors/index.js

@@ -1,2 +1,4 @@
+'use strict';
+
 module.exports = require('./DJSError');
 module.exports.Messages = require('./Messages');