Avoid recursively setting `KEY__SHADOW_RESOLVER`

[nolanlawson]

As described in #4088, we have a function called recursivelySetShadowResolver:

lwc/packages/@lwc/synthetic-shadow/src/faux-shadow/shadow-token.ts

Lines 49 to 56 in 55112b7

	function recursivelySetShadowResolver(node: Node, fn: any) {
	(node as any)[KEY__SHADOW_RESOLVER] = fn;
	const childNodes = childNodesGetter.call(node);
	for (let i = 0, n = childNodes.length; i < n; i++) {
	recursivelySetShadowResolver(childNodes[i], fn);
	}
	}

This is called at the top level for all static fragments. For non-static fragments, we set it on all nodes in linkNodeToShadow:

lwc/packages/@lwc/engine-core/src/framework/rendering.ts

Lines 526 to 535 in 55112b7

	function linkNodeToShadow(elm: Node, owner: VM, renderer: RendererAPI) {
	const { renderRoot, renderMode, shadowMode } = owner;
	const { isSyntheticShadowDefined } = renderer;
	// TODO [#1164]: this should eventually be done by the polyfill directly
	if (isSyntheticShadowDefined) {
	if (shadowMode === ShadowMode.Synthetic || renderMode === RenderMode.Light) {
	(elm as any)[KEY__SHADOW_RESOLVER] = renderRoot[KEY__SHADOW_RESOLVER];
	}
	}
	}

This serves two purposes:

1. isNodeShadowed/isNodeFromTemplate – legacy API used by Locker
2. "portals" aka MutationObserver – the patched MutationObserver has to know which nodes come from synthetic shadow roots

This could be much more efficient, though, if we computed it on-demand. Rather than traversing through the whole DOM tree to set this value on every node (not just elements – nodes!), we could compute it on-demand based on the parent/host of the node.

This would 1) restrict the cost to only when isNodeShadowed/MutationObserver is invoked, and 2) be more efficient in general, since walking up the tree is faster than traversing down the tree.

OTOH we would probably have to do some caching to avoid recomputing this for expensive MutationObserver calls.