feat(cli): add TUI REPL with syntax highlighting

[eddmann]

Summary

Add an interactive TUI (terminal user interface) REPL for santa-lang with enhanced editing and visualization features.

Description

Create a terminal-based REPL that goes beyond a simple line-by-line input, providing a rich interactive environment for exploring and experimenting with santa-lang code.

Proposed Features

Enhanced Input

• Multi-line editing with proper cursor movement
• Syntax highlighting as you type
• Auto-indentation for blocks and nested expressions
• Parenthesis/bracket matching and highlighting
• History navigation with search (Ctrl+R)

Auto-completion

• Built-in function names
• User-defined variables and functions in scope
• Keyword completion (let, match, if, etc.)

Output Display

• Pretty-printed results with syntax highlighting
• Truncated output for large data structures with expand option
• Type information display (optional)

Additional Features

• :help command for built-in documentation
• :type <expr> to show expression type
• :clear to reset the environment
• :load <file> to load and evaluate a file
• :save <file> to save session history

Example Session

santa> let numbers = [1, 2, 3, 4, 5]
[1, 2, 3, 4, 5]

santa> numbers
  |> map(|x| x * 2)
  |> filter(|x| x > 4)
[6, 8, 10]

santa> :type numbers
List<Integer>

santa> :help map
map(fn, list) - Transforms each element in a list using the provided function

Implementation Considerations

• Use a TUI library (e.g., ratatui for Rust, blessed/ink for Node)
• Maintain REPL state/environment across inputs
• Handle Ctrl+C gracefully (cancel current input, not exit)
• Support terminal resizing
• Consider vi/emacs keybinding modes

References

• ipython - Enhanced Python REPL
• evcxr - Rust REPL with rich features
• nushell - Modern shell with TUI features