Interactivity API: Improve internal deepMerge function

[DAreRodz]

What?

Improves the internal deepMerge() function implementation. The main reason for these changes is to ensure deepMerge() follows the following rules:

1. It doesn't create signals when manipulating state proxies.
2. It updates signals only if they exist.
3. It batches all signal updates together.

Why?

Required by the upcoming getServerState() and getServerContext() functions to handle state updates from the server.

How?

The new deepMerge() implementation directly manipulates the objects behind state proxies. If an existing property value is updated, it checks whether a corresponding PropSignal instance exists. Only in that case are the signal values updated.

Testing Instructions

I added unit tests for the three rules mentioned above.

[luisherranz]

This looks fantastic. Thanks, David! I know you haven't finished yet, but I'm going to approve it anyway.

[github-actions]

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Co-authored-by: DAreRodz <darerodz@git.wordpress.org>
Co-authored-by: luisherranz <luisherranz@git.wordpress.org>

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[DAreRodz]

Thanks for the review, @luisherranz! I've addressed your feedback and polished the code a bit. It should be ready now. 🙂