Add basic C++ syntax highlighting to FileViewer

[alexisM8]

This PR introduces basic syntax highlighting support for C++ source files in FileViewer.

Features:

• Token-based lexer for C++ (keywords, strings, comments, numbers, preprocessor)
• Support for single-line (//) and multi-line (/* */) comments with stateful lexing
• Custom rendering using ImGui DrawList for colored text

Notes:

• Lexer is line-based with minimal state (LexState) carried across lines
• Only .cpp/.hpp-style files are currently supported
  • any file will be syntax-highlighted it just will look bad if its not a c-sytle language

Tested on:

• .cpp / .hpp files
• Mixed comment styles
• Large files (basic performance)

Future improvements (optional):

• Token caching
• More complete C++ parsing
• Gutter interaction refinements

Before:

After:

ps: very cool project :)