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/eslint/lib/rules/utils/unicode/is-combining-character.js

@@ -0,0 +1,13 @@
+/**
+ * @author Toru Nagashima <https://github.com/mysticatea>
+ */
+"use strict";
+
+/**
+ * Check whether a given character is a combining mark or not.
+ * @param {number} codePoint The character code to check.
+ * @returns {boolean} `true` if the character belongs to the category, any of `Mc`, `Me`, and `Mn`.
+ */
+module.exports = function isCombiningCharacter(codePoint) {
+    return /^[\p{Mc}\p{Me}\p{Mn}]$/u.test(String.fromCodePoint(codePoint));
+};