Support for svdvals

[MikeInnes]

We already have support for this via svd(X).S, but support for svdvals directly would be nice. It'd also be great to support GenericLinearAlgebra, which I don't think we do at the moment.

[oxinabox]

GenericLinearAlgebra

The intent is that ChainRules.jl does not add rules for packages into ChainRules itself.
Those rules belong in the packages themselves.
Packages depend on ChainRulesCore for that.

We have an exception right now for SpecialFunctions and NaNMath, just because they were in DiffRules, though we plan to move them out eventually.

This is not the issue to talk about that as a matter of policy in though.

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we should add svdvals support yes, PRs welcome.