add 'ᵀ postfix operator for transpose

[simeonschaub]

I feel like transpose is really the only sensible meaning of 'ᵀ and this was also the main use case for #37247, so I think defining it in Base makes the most sense.
fixes JuliaLang/LinearAlgebra.jl#410

[pablosanjose]

Why was ᵀ discarded? It seems cleaner to me than 'ᵀ.

[simeonschaub]

It's already a valid identifier and identifiers like aᵀb are currently quite often used as variable names.

[simeonschaub]

Triage approved of this, so should be ready to go.

[GiggleLiu]

Just read the 1.6 news, to me, it definitely looks like a conjugate rather than a transpose.

[dnadlinger]

Just saw this in the 1.6 changelog too, and to me (math/physics at ETH Zurich), a'ᵀ pretty "unambiguously" parses as transpose(adjoint(a)), i.e. conj(a) as well – except that, of course, that's not what it means as per this PR!

Imho 1.6 should be released without this, and a discussion reopened later.

[Roger-luo]

I didn't see this change until now either, I second what @GiggleLiu @dnadlinger says a'ᵀ does look more like transpose+adjoint.

[dnadlinger]

I've opened #40070 for further discussion.