Have `insupport` check that the input is scalar

[theogf]

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
SciML/Static.jl#64 (comment)

Originally posted by @cscherrer in #56 (comment)