Inconsistent behavior for `keypress` event for surrogate pairs

[jrandolf-google]

In all three browsers, we have inconsistent behavior w.r.t. keypress events with unicode characters that are surrogate pairs.

Specifically, the following behavior is observed:

• Chrome - No event is emitter.
• Firefox - Two keypress events are emitted, one for each surrogate pair.
• Safari - One keypress event is emitted, for the full U32 unit.

The user expectation is probably aligned with Safari's rather than Firefox's, so I propose we align with Safari on this matter.

CC: @whimboo

[mathiasbynens]

I think the ideal place to discuss this is upstream: https://github.com/w3c/uievents/issues/346