Initial commit: Developer Tools MVP with visual editor

- Complete React app with 7 developer tools
- JSON Tool with visual structured editor
- Serialize Tool with visual structured editor
- URL, Base64, CSV/JSON, Beautifier, Diff tools
- Responsive navigation with dropdown menu
- Dark/light mode toggle
- Mobile-responsive design with sticky header
- All tools working with copy/paste functionality

node_modules/define-lazy-prop/index.d.ts

@@ -0,0 +1,33 @@
+/**
+Define a [lazily evaluated](https://en.wikipedia.org/wiki/Lazy_evaluation) property on an object.
+
+@param object - Object to add property to.
+@param propertyName - Name of the property to add.
+@param fn - Called the first time `propertyName` is accessed.
+
+@example
+```
+import defineLazyProp = require('define-lazy-prop');
+
+const unicorn = {
+	// …
+};
+
+defineLazyProp(unicorn, 'rainbow', () => expensiveComputation());
+
+app.on('user-action', () => {
+	doSomething(unicorn.rainbow);
+});
+```
+*/
+declare function defineLazyProp<
+	ObjectType extends {[key: string]: unknown},
+	PropertyNameType extends string,
+	PropertyValueType
+>(
+	object: ObjectType,
+	propertyName: PropertyNameType,
+	fn: () => PropertyValueType
+): ObjectType & {[K in PropertyNameType]: PropertyValueType};
+
+export = defineLazyProp;