fix: make repl support multi-line code

[martian56]

This pull request introduces improvements to the Raven REPL, enabling robust multi-line input by tracking delimiter balance and providing a better user experience. The main changes include a new delimiter_depth utility, a refactored REPL input loop, and supporting code organization updates.

REPL enhancements:

• Added a new delimiter_depth function in src/repl.rs to accurately count unclosed parentheses, braces, and brackets outside of strings and comments, supporting multi-line input in the REPL. Includes comprehensive unit tests.
• Refactored the REPL in src/main.rs to use a new read_repl_snippet function, which reads user input until all delimiters are balanced, allowing for multi-line code entry. The prompt now changes to ...> for continued input.
• Improved REPL command handling with a new ReplInput enum, providing clearer handling of exit, quit, help, and code input.

Code organization:

• Added a new repl module in src/lib.rs to encapsulate REPL-related helpers.
• Updated imports in src/main.rs to use the new delimiter_depth function from the repl module.

Other:

• Bumped the crate version in Cargo.toml from 1.6.3 to 1.6.4 to reflect these improvements.

Closes #13