type stability: `(::Polynomial)(::ImmutablePolynomial)` composition yields `ImmutablePolynomial`

With v4.0.0:
```julia-repl
julia> using Test, Polynomials

julia> p = ImmutablePolynomial((1,))
ImmutablePolynomial(1)

julia> q = Polynomial([10, 20])
Polynomial(10 + 20*x)

julia> @inferred q(p)
ERROR: return type Polynomials.ImmutableDensePolynomial{Polynomials.StandardBasis, Int64, :x, 1} does not match inferred return type Any
```

It doesn't make sense for this composition to return an `ImmutableDensePolynomial`, because that makes type stability impossible, because the degree of `q` is not available in the type domain.