Improve typehinting for Item value

[gapple]

The Item class' value property (via TupleTrait) is typehinted to mixed, but it should more specifically be bool|float|int|string|Bytes|Date|DisplayString|Token, though it would be a little simpler if the classes that decorate special types had a common interface or base class.

I ran into some quirks getting PhpStorm to understand either a phpstan global alias or a local alias on the Item class that was imported elsewhere.