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/stylehacks/src/plugins/leadingUnderscore.js

@@ -0,0 +1,51 @@
+'use strict';
+const BasePlugin = require('../plugin');
+const { IE_6 } = require('../dictionary/browsers');
+const { PROPERTY } = require('../dictionary/identifiers');
+const { DECL } = require('../dictionary/postcss');
+
+/**
+ * @param {string} prop
+ * @return {string}
+ */
+function vendorPrefix(prop) {
+  let match = prop.match(/^(-\w+-)/);
+  if (match) {
+    return match[0];
+  }
+
+  return '';
+}
+
+module.exports = class LeadingUnderscore extends BasePlugin {
+  /** @param {import('postcss').Result=} result */
+  constructor(result) {
+    super([IE_6], [DECL], result);
+  }
+
+  /**
+   * @param {import('postcss').Declaration} decl
+   * @return {void}
+   */
+  detect(decl) {
+    const { before } = decl.raws;
+
+    if (before && before.includes('_')) {
+      this.push(decl, {
+        identifier: PROPERTY,
+        hack: `${before.trim()}${decl.prop}`,
+      });
+    }
+
+    if (
+      decl.prop[0] === '-' &&
+      decl.prop[1] !== '-' &&
+      vendorPrefix(decl.prop) === ''
+    ) {
+      this.push(decl, {
+        identifier: PROPERTY,
+        hack: decl.prop,
+      });
+    }
+  }
+};