Add webdriver_key_to_key_code() in Internals.cpp to properly translate
WebDriver special key codes (0xE000-0xE05D) to KeyCode values with
appropriate modifiers. This ensures keys like Enter, Backspace, and
arrow keys are handled correctly when sent via Internals::send_text().
In EventHandler::handle_keydown(), strip Mod_Keypad when determining
Enter key behavior since it only indicates key location (numpad vs
standard keyboard), not a behavior change. The modifier is still passed
through to KeyboardEvent for the location property.
This gains us 656 WPT subtest passes in `editing`.
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