fix when LHS of a reactive assignment is a member expression

[Conduitry]

Fixes #4212. In DOM mode, we were declaring the variable automatically, which is almost certainly never going to be helpful (and broke stuff like using it to update document.title). In SSE mode, we were writing actually invalid code (let document.title = ...). This simply skips the automatic declaration of the variable when the LHS of the assignment is a MemberExpression.

[Conduitry]

Hmm this isn't quite the right solution, as it doesn't handle this (wacky) situation:

$: ({ key: document.title } = foo);

I'm not sure what the best way to handle this is. If we wanted to add an option for extract_names to skip names used in MemberExpressions, that would have to go in periscopic, and suddenly this isn't so quick to fix anymore.

[antony]

This reminds me of this issue: #3310

Encouraging operations on document to be made by a <svelte:document> tag might be a good way to get around complexities such as this.

[Conduitry]

Regardless of <svelte:document> I still think we should do something about this.

I've decided to just merge this as-is, which should cover the majority of cases that actually occur in real life. I'll open an issue to discuss the new feature over in periscopic's repo, and then eventually we can fix this properly here.