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/v8-to-istanbul/lib/line.js

@@ -0,0 +1,34 @@
+module.exports = class CovLine {
+  constructor (line, startCol, lineStr) {
+    this.line = line
+    // note that startCol and endCol are absolute positions
+    // within a file, not relative to the line.
+    this.startCol = startCol
+
+    // the line length itself does not include the newline characters,
+    // these are however taken into account when enumerating absolute offset.
+    const matchedNewLineChar = lineStr.match(/\r?\n$/u)
+    const newLineLength = matchedNewLineChar ? matchedNewLineChar[0].length : 0
+    this.endCol = startCol + lineStr.length - newLineLength
+
+    // we start with all lines having been executed, and work
+    // backwards zeroing out lines based on V8 output.
+    this.count = 1
+
+    // set by source.js during parsing, if /* c8 ignore next */ is found.
+    this.ignore = false
+  }
+
+  toIstanbul () {
+    return {
+      start: {
+        line: this.line,
+        column: 0
+      },
+      end: {
+        line: this.line,
+        column: this.endCol - this.startCol
+      }
+    }
+  }
+}