Add error hint for incorrect stacked indexing (#40934)
A common entry level mistake is to try to index a matrix with two sets
of brackets, e.g. `a = [1 2; 3 4]; a[1][2] = 5`
This will lead to an error that `setindex!()` on the element type of `a`
is missing.
This PR adds an error hint for the case where a MethodError is raised
when `setindex!` is called with a `Number` as the first argument.
I considered going broader than numbers, but it seems more likely that
this kind of mistake would happen when working with simple number arrays
vs. something more advanced.
Could also consider if it is possible to do the same for when
`getindex()` is called on a `Number`, which emits a BoundsError.
Co-authored-by: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
