Add method `hermitianpart`

[kbarros]

The docs for LinearAlgebra.hermitianpart state that the behavior should be:

hermitianpart(A) = (A + A') / 2

Would it be possible to add an implementation of this method for AbstractPolynomialLike or similar? It seems natural to have since the adjoint method on polynomials already exists.

[kbarros]

A counter-argument would be that the method is not defined on scalars either:

using LinearAlgebra
hermitianpart(1 + 2im)

For our use case, the "hermitianpart" notation would be convenient because we interpret the polynomial variables to be a symbolic representation of commuting operators:
SunnySuite/Sunny.jl#388

[blegat]

I see that in JuliaLang/julia#31836 they decided to not implement it for Number but rather just use _hermitianpart(::Number). I feel that they could have just used hermitianpart(::Number) there and I don't have any objection to implement it in MultivariatePoynomials. PR welcome to add it next to

MultivariatePolynomials.jl/src/operators.jl

Line 438 in 2f04595

LinearAlgebra.ishermitian(p::AbstractPolynomialLike) = p == conj(p)