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-surrogate-pair.js

@@ -0,0 +1,14 @@
+/**
+ * @author Toru Nagashima <https://github.com/mysticatea>
+ */
+"use strict";
+
+/**
+ * Check whether given two characters are a surrogate pair.
+ * @param {number} lead The code of the lead character.
+ * @param {number} tail The code of the tail character.
+ * @returns {boolean} `true` if the character pair is a surrogate pair.
+ */
+module.exports = function isSurrogatePair(lead, tail) {
+    return lead >= 0xD800 && lead < 0xDC00 && tail >= 0xDC00 && tail < 0xE000;
+};