Have `insupport` check that the input is scalar

There is no way to know if the input is scalar since we want to be able to work with other types like `Static` or symbolic values.

See:

> I don't know of a good way to check this, since `x` might be a symbolic or static value that's not `<:Real`. For example, see
https://github.com/SciML/Static.jl/pull/64#issuecomment-1144979944

_Originally posted by @cscherrer in https://github.com/cscherrer/MeasureBase.jl/pull/56#discussion_r889038270_