[Fix]: Flaky Keyboard module

[thalissonvs]

The keyboard module has critical limitations preventing full keyboard control functionality. Letter keys are missing from the Key enum, and key_down()/key_up() methods are only available on WebElement, not on Tab.

We need to add KEY_A through KEY_Z with their corresponding virtual key codes (65-90). This is preventing using keyboard combinations like Ctrl+A, Ctrl+C, etc.

Solution

1. Add letter keys to the Key enum in pydoll/constants.py following the same pattern as existing keys
2. Implement key_down() and key_up() methods in the Tab class, similar to how press_keyboard_key() is already implemented
3. Add unit and integration tests

This will unblock the keyboard control documentation which is currently marked as "Under Construction".