error on expression scope store

[tanhauhau]

Fixes #5048

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Tests

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[Conduitry]

It looks like this would also prevent using $store in a scope where the top-level store had been shadowed, correct? I don't really have a strong opinion about whether that's something we want. Using $store where store has been shadowed is confusing, but it currently works fine. It's only assignments to $store - which get rewritten to attempt a direct reference to store - that cause a problem.

I guess this partially comes down to whether we ever plan to let you autosubscribe to stores that aren't declared on the top level. Making $store refer to the non-top-level store would technically be a breaking change, although the current behavior here might not be officially documented anywhere, and might not be desirable. If we do want to eventually change this behavior, it's probably better to have an intervening period where the input causes a compiler error, rather than continuing to permit this and then suddenly changing its behavior sometime in the future.

As I've been writing this, I've sort of talked myself into thinking that disallowing all uses of $store when store is shadowed is the better way forward, but I wanted to make sure everyone's okay with that, even though the compiler would then reject some currently working (albeit confusing) components.

[tanhauhau]

I would think that any variable name that begins with $ refers to the store value of the variable name without the $. ie: $store refers to store value of store, and store should be the one in the current scope.

[Conduitry]

Looking at this PR now, I think we want to also prevent usage of $store in the <script> block when the top-level store has been shadowed. For example,

<script>
	import { writable } from 'svelte/store';
	const store = writable();
	{
		let store;
		$store = 42;
		console.log($store);
	}
</script>

produces a runtime error, but should also be a compile-time error.

Similarly to the previous comment, this would also have the effect of making the following code:

<script>
	import { writable } from 'svelte/store';
	const store = writable(42);
	{
		let store;
		console.log($store);
	}
</script>

stop printing '42' and start being a compile-time error, which seems like a reasonable enough slight change if we're also okay with that happening in the template.

[tanhauhau]

That make sense to me.