[w3c/uievents] Redefine when `keypress` event should be fired (#266)

Currently, `keypress` event is defined as that it's fired only when a key press introduces a character.

https://w3c.github.io/uievents/#event-type-keypress
> If supported by a user agent, this event MUST be dispatched when a key is pressed down, if and only if that key normally produces a character value

However, both Chrome and Safari violates this rule, but [some web apps may depend on control character inputting `keypress`](https://bugzilla.mozilla.org/show_bug.cgi?id=1602694). So, there should be clearer and not breaking existing web apps rule.

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