[ENHANCEMENT] Introduce `get_type_parameters` to access unspecified type parameters

[mtfishman]

For example:

get_type_parameters(Matrix) == (TypeVar(:T), 2) # This equality technically doesn't hold because `TypeVar(:T) ≠ TypeVar(:T)`
get_type_parameters(Matrix, 1) == TypeVar(:T) # This equality technically doesn't hold because `TypeVar(:T) ≠ TypeVar(:T)`
get_type_parameters(Matrix, 1, nothing) == nothing # Set a default value to use if unspecified

This would act as a public API for the private functionality TypeParameters.parameters, and serves as an unchecked version of type_parameters.

[mtfishman]

Probably once we do this we should add support for setting type parameters to TypeVar, which effectively unspecifies the type parameter, but also would support setting parameters with type constraints, i.e.:

set_type_parameters(Matrix{Float64}, eltype, TypeVar(:T, Real)) === Matrix{<:Real}