LibWeb: Fire input event on user interaction with input element buttons

[tcl3]

An input event is now fired when the step up or step down button of an input element of type number is clicked.

This ensures that any associated <output> element is updated when these buttons are clicked.

Demo:

https://github.com/SerenityOS/serenity/assets/2817754/86b44f2a-1a26-411f-947f-d03a4d9f62b4