linalg: add generalized eigenvalue problem to `eig`

[perazz]

Motivation

I'm opening a issue to discuss the addition of the generalized eigenvalue problem:

A⋅x=λ⋅B⋅x

as an extension of eig, that currently only handles the standard eigenvalue problem:

A⋅x=λ⋅x

Prior Art

Scipy is the only package that offers this option

Additional Information

Fortran Discourse thread where the feature was recently requested