Add support for ProductNamedTupleDistribution

[sunxd3]

JuliaStats/Distributions.jl#1803 introduced NamedTupleVariate.

The same PR also added ProductNamedTupleDistribution which is a product distribution that returns NamedTuple typed data when calling rand. For ProductNamedTupleDistribution, all the components are independent from each other. An example of it is Distributions.ProductNamedTupleDistribution.

We might want to add support for ProductNamedTupleDistribution and other NamedTupleVariate (that we and others define). This would involve making changes to the syntax, essentially introducing a stochastic version of unpacking.

For instance,

@model function demo()
    x, y ~ product_distribution((x=Normal(), y=Dirichlet([2, 4])))
    return x, y
end

I think updating model macro to allow Expr(:tuple, ...) on the LHS shouldn't be too hard. However it might cause some trouble with the tilde pipeline (@mhauru).

We should also think of some corner cases to avoid complications similar to dot-tilde.