Add opt-in Event, EventTarget, CustomEvent, AbortController and AbortSignal

[lahma]

Part of the opt-in web API series (#3079): Event, CustomEvent, EventTarget, AbortController and AbortSignal behind WebApiFeatures.Events (in Default). These are prerequisites for fetch's cancellation model. Usual rules: BCL only, net8+, opt-in, default engine unchanged.

Dispatch is flat, and deliberately runs both passes. The DOM standard's dispatch walks an event path built from a node tree; author-created EventTargets have no tree, so the path is the single item «target» — and everything that survives that reduction is the real algorithm, not an approximation: both passes still run with eventPhase at AT_TARGET, so capture decides which pass a listener runs in (a capturing listener on a flat target runs before a non-capturing one whatever order they were registered — the standard's own behavior, pinned); stopPropagation ends the dispatch after the current pass while stopImmediatePropagation also breaks out of it; each pass walks a clone so a listener added mid-dispatch does not run and one removed does not either; once listeners are removed before invocation so a throwing callback cannot run twice.

A listener that throws erupts from dispatchEvent — the spec's "report the exception" presumes a reportError channel that does not exist yet, and swallowing would lose the error entirely on a console-less engine; so it propagates, matching the timer-callback contract, with dispatch state unwound in a finally. When reportError lands this becomes report-and-continue behind the sink.

AbortSignal is fetch's cancellation seam. Abort algorithms run before the abort event — the engine's own algorithm (cancelling the signal's lazily-created CancellationTokenSource) runs first of all, so a CLR-side operation stops before any script observes the abort. AbortSignal.timeout(ms) schedules on the timer queue (fires only while pumped, counts against MaxActiveTimers, [EnforceRange] semantics); AbortSignal.any implements dependent signals with source flattening and link-dropping on abort so an aborted signal retains nothing; addEventListener's signal option unregisters on abort, and removeEventListener also takes the algorithm off the signal — avoiding a retention the spec itself leaves behind. The events feature creates the per-engine web-API state even without the timers flag, because Event.timeStamp reads the shared time origin and AbortSignal.timeout needs the queue.

162 new tests. Documented WebIDL simplifications follow the series' precedent (non-enumerable prototype operations; strong rather than weak dependent-signal lists, with the trade-off explained on the type). Verified: all-TFM build, both suites both frameworks, both host-contract-verification configurations.

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