Support shared signals in Preact islands

[matthewp]

Changes

• This makes it possible to share state between Preact components rendered to islands. Signals can be created in your Astro componnents and passed to islands. They will be serialized to HTML and in the client recreated as shared signals.

Testing

• Test added. Key thing about this test is checking that the keys made for the signals are the same between the islands.

Docs

N/A