feat(effectScope): add lazy-initialized signal getter with automatic abort on stop

[9romise]

This PR introduces a lazy-initialized signal getter on EffectScope.

• When scope.signal is accessed for the first time, an AbortController will be created and its AbortSignal returned.
• If scope.signal is never accessed, no controller will be created, avoiding unnecessary overhead.
• When scope.stop() is called, if an AbortController exists, it will automatically trigger controller.abort().

Related: #13861 (comment), #13861 (comment)

It would be good to have this for effectScope and watch. This would allow us to get the signal with getCurrentScope.

const {signal} = getCurrentScope()

fetch(url, {signal})

/cc @ferferga, @OrbisK, @antfu

[OrbisK]

Someday, we could follow up on this by merging the effects signal with its parent signals (AbortSignal.any()), which would remove the need for the cleanup array + loop. (Should be benchmarked)

core/packages/reactivity/src/effectScope.ts

Lines 151 to 154 in 179b397

	for (i = 0, l = this.cleanups.length; i < l; i++) {
	this.cleanups[i]()
	}
	this.cleanups.length = 0