Deferred trigger prerequisite refactors

[crisbeto]

These changes include some refactors to facilitate the implementation of the viewport, interaction and hover triggers. The majority of the changes is around reworking the R3TargetBinder to consider if, switch and defer blocks (plus the sub-blocks) as separate embedded views. This allows us to properly resolve references within their scope and is consistent with how they behave at runtime. Split up across the following commits to make it easier to review:

refactor(compiler): require a reference in interaction and hover triggers

Updates the parsing for interaction and hover triggers to require a reference to an element.

refactor(compiler): update binder to account for new semantics

When the TargetBinder was written, the only embedded-view-based nodes were templates, but now we have {#if}, {#switch} and {#defer} which have similar semantics. These changes rework the binder to account for the new nodes.

refactor(compiler): add utility to resolve the deferred block trigger element

Adds a utility to the BoundTarget that helps with resolving which element a deferred block is pointing to. We need a separate method for this, because deferred blocks have some special logic for where the trigger can be located.

[AndrewKushnir]

@crisbeto looks great 👍

[AndrewKushnir]

nit:

Suggested change

	const scopedNodeEntities = extractScopedNodeEntities(scope);
	const scopedNodeEntities = extractScopedNodes(scope);

[crisbeto]

I think the current name is more accurate, because we aren't extracting the scoped nodes themselves, but rather entities (e.g. Reference and Variable) present in them.

[AndrewKushnir]

Suggested change

	nestingLevel, scopedNodeEntities, usedPipes, eagerPipes, deferBlocks);
	nestingLevel, scopedNodes, usedPipes, eagerPipes, deferBlocks);

[AndrewKushnir]

Since we use this check in a couple places, it'd be great to extract it to a separate fn:

Suggested change

	} else if (
	nodeOrNodes instanceof DeferredBlock || nodeOrNodes instanceof DeferredBlockError ||
	nodeOrNodes instanceof DeferredBlockPlaceholder ||
	nodeOrNodes instanceof DeferredBlockLoading) {
	} else if (isScopedNode(nodeOrNodes)) {

We can check all node types there (including IfBlockBranch and ForLoopBlock), so we can do do type narrowing too:

function isScopedNode(node: unknown): node is ScopedNode { ... }

[crisbeto]

I agree that this felt a bit weird when I was writing it, but there are a couple of reasons why I think we should keep it:

1. Once template type checking is implemented for control flow, there will be different handling for each ScopedNode, with the exception of the deferred ones.
2. Using the if/else if like we do here will result in an error whenever we add a node type to ScopedNode, which is good because it makes it easier to see where we need to adjust the logic. In comparison, if we do something like following we won't get such an error:

if (!isScopedNode(node)) {
  return ...;
}

if (node instanceof IfBlock) {
   ...
} else if (node instanceof DeferredBlock) {
   ...
}

[pkozlowski-opensource]

This PR was merged into the repository by commit d538908.

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