Remove Object.assign in node input / output slot classes

This pattern was implemented around 6 months ago, allowing inputs / outputs to be recreated as concrete classes without breaking extensions.

- Ref: Comfy-Org/litegraph.js#476

Unfortunately, it is also extremely brittle, and bypasses type checking. Any enumerable properties not explicitly excluded are simply copied to the new concrete object without any validation.

N.B. New code must handle properties correctly, rather than rely on this workaround.

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-4699-Remove-Object-assign-in-node-input-output-slot-classes-2476d73d365081f0a575ea5bea6c1e1c) by [Unito](https://www.unito.io)