wrong printing for single-element matrix

[simeonschaub]

The following printing is wrong, as the output would produce a vector, not a matrix:

julia> show(IOContext(stdout, :compact=>true), fill(42, 1, 1))
[42]

Observed on master and 1.5.

[StefanKarpinski]

The trouble is we don't have an input syntax for single-column matrices.

[simeonschaub]

We could probably just print this as fill(42, 1, 1). I don't think there is a more concise way to otherwise express this.

[JeffBezanson]

Related: #28905

[simeonschaub]

@thofma and @mbauman seemed against this in #36737, but I still think it would be good to be able to distinguish matrices from vectors when printed inside arrays. Perhaps a compromise would be printing matrices with a trailing semicolon, for example [42;], although this would of course still produce a vector, so might be more confusing.

[simeonschaub]

This is now fixed by #33697