Add REPL improvements

[dev-dami]

Description

The REPL (carv repl) needs quality-of-life improvements.

Suggested Features

• Command history (up/down arrows)
• Multi-line input for functions/classes
• :help command showing available commands
• :type <expr> to show type of expression
• :clear to clear screen
• :load <file> to load a file into REPL
• Syntax highlighting in REPL output
• Tab completion

Where to Start

• cmd/carv/main.go - REPL implementation
• Consider using a library like readline for history

Good First Issue

Start with one feature at a time. History and :help are good starting points!