feat(keymap_derive): support custom key symbols and help messages

[rezigned]

Summary

Add support for custom key symbols and help text in keymap attribute definitions.

Changes:

• Extended #[key(...)] attribute to accept symbol = "..." and help = "..." named arguments
• Added symbol and help fields to Item struct with builder methods (with_symbol(), with_help())
• Symbol defaults to first key binding if not explicitly provided
• Added tests verifying symbol/help parsing and fallback behavior

Example usage:

#[key("ctrl-b", symbol = "^B", help = "jump")]
Jump