[feat] dispatch cancelable custom events

[bluwy]

Fixes #4623

Dispatching custom events can be cancelled.

const dispatch = createEventDispatcher()

const result = dispatch('foo', 'detail', { cancelable: true })
// return false if canceled via `e.preventDefault()`, otherwise always true

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Tests

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[PuruVJ]

TQSM for this PR. Can you specify how we can detect whether an event was cancelled? As far as I know, you need the event object to detect event.defaultPrevented. Am I missing something?

[bluwy]

Good point. dispatch should probably return a boolean, similar to dispatchEvent, would need to handle that too.

[bluwy]

Updated to return a boolean depending on if it's canceled via preventDefault() or not. I tried to return an event instead, but the code doesn't always emit an event, so it's not always possible.

[bluwy]

Updated docs, changelog and fixed the types. This should be good to go (unless there's some wording changes).

[bluwy]

Conduitry, I've requested your review in case you want to take a look at this. In the last maintainer's meeting, we had agreed that this would be a safe addition to the API.