feat: add generic modifier support via CSI u escape sequences

[menny]

This PR adds support for modified special keys (Shift, Ctrl, Alt, Meta) by implementing the standard CSI u (Unicode keyboard input) protocol.

Specifically, it handles:

• Shift+Enter (e.g. for newlines without submission in modern TUIs like gemini-cli)
• Modified Tab, Backspace, and Escape.

The implementation hooks into attachCustomKeyEventHandler to intercept raw keyboard events and map them to standard escape sequences (e.g. \x1b[13;2u for Shift+Enter).

Includes unit tests for the modifier mapping logic.

[menny]

This is useful for me as some of the CLIs I use over ssh have special handling for SHIFT + ENTER.