Refactor and improve NamedTuple deserialization from JSON and YAML

[HertzDevil]

• Makes the temporary value variables non-nilable unless their corresponding types are nilable. This saves the downcasts at the end of deserialization.
• Fixes compilation error for deserialization of NamedTuples that use file-private value types.
• Fixes compilation error for deserialization of empty NamedTuples.

[straight-shoota]

I'm wondering if it wouldn't be easier to always use an uninitailized value and keep track if it was found. And only apply nil handling for nilable properties when constructing the final tuple.

[straight-shoota]

Wouldn't this work?

Suggested change

	%var{key.id} = uninitialized typeof(begin
	value = uninitialized self
	value[{{ key.symbolize }}]
	end)
	%var{key.id} = uninitialized {{ type }}

[asterite]

I think we should put a comment around why we do it this way. I think I made the exact same comment in a previous PR but now I can't remember why doing it the way @straight-shoota (and previously me) suggest is not a good idea.

[HertzDevil]

It's #11834. But there is an even more idiomatic way of expressing this: #12011

[sdogruyol]

Thank you @HertzDevil 🙏