Proposal for moving StatsBase weights framework to stdlib/Statistics

[Nosferican]

The proposal is to migrate the weights framework from StatsBase.jl to Statistics since it goes very tightly with that code. It would also allow the StatsModels pipeline to drop StatsBase in favor of StatsModels (statistical abstraction is being moved there).

[nalimilan]

For those not familiar with this code:

• the weights types : http://juliastats.github.io/StatsBase.jl/latest/weights.html
• the weighted mean, var and std using these: http://juliastats.github.io/StatsBase.jl/latest/scalarstats.html

[tamasgal]

Just an update of the documentation URLs:

• the weights types: https://juliastats.org/StatsBase.jl/stable/weights/
• the weighted mean, var and std using these: https://juliastats.org/StatsBase.jl/stable/scalarstats/

[nalimilan]

JuliaLang/julia#31395 does most of the work but it's been blocked on design issues regarding the weighted sum, and more generally about whether a weights keyword argument should be used vs. a weighted object.