[threads] Simplify and generalize heap type writing without GC (#6765)

We represent `ref.null`s as having bottom heap types, even when GC is
not enabled. Bottom heap types are a feature of the GC proposal, so in
that case the binary writer needs to write the corresponding top type
instead. We previously had separate logic for this for each type
hierarchy in the binary writer, but that did not handle shared types and
would not have automatically handled other new types, either. Simplify
and generalize the implementation and test that we can write `ref.null`s
of shared types without GC enabled.

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