Properly dispatch keypress event
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This was an attempt to fix #14659. It turned out that the problem wasn't what I thought it was. So, I didn't fix that. On the brighter side, this fixes two related issues.
- Previously, we were unable to launch `keypress` events from `input` and `textarea` elements, because [we'd been cancelling](https://github.com/servo/servo/blob/1327ebd52f53f5f6637a12fab6cf0cad0aa0be6f/components/script/dom/htmlinputelement.rs#L1120-L1124) the key events, so that they don't trigger window navigation - #8400). I've introduced an enum to represent an additional state to an event's cancellation.
- [According to the spec](https://w3c.github.io/uievents/#keypress-event-order), `keypress` (if available) should be dispatched immediately after `keydown`, and it should be followed by `input`. Canceling `keypress` should also cancel `input`. But, we'd been dispatching `input` before `keypress`. We now dispatch `input` once the `keypress` event is on the respective elements.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
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- [x] These changes do not require tests because it's a refactor?
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r? @jdm or anyone interested
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