Add rgx — terminal regex debugger

[brevity1swos]

Description

Add rgx to the Productivity and Utilities section.

rgx is a terminal regex debugger built with ratatui + crossterm that provides:

• Real-time matching with 3 regex engines (Rust regex, fancy-regex, PCRE2)
• Capture group highlighting with named group support
• Plain-English pattern explanations (regex-syntax AST walker)
• Replace/substitution mode with live preview
• Pattern syntax highlighting (Catppuccin palette)
• Undo/redo, pattern history, clipboard copy
• Mouse support (click-to-focus, scroll)
• Whitespace visualization
• Multi-page context-sensitive cheat sheet

Published on crates.io as rgx-cli and available via Homebrew.