Add diagonal as a generalization of LinearAlgebra.Diagonal

[lkdvos]

This adds a utility entrance-point to turn a vector into a diagonal matrix, without implying the return type. Specifically, it adds diagonal, which relates to Diagonal in the same way that adjoint relates to Adjoint.
The main purpose is to have the ability to write generic code, without having to force everything to be of type Diagonal.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.37%. Comparing base (2bd0713) to head (99e3475).
Report is 1 commits behind head on main.

Additional details and impacted files

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+ Coverage   77.20%   77.37%   +0.16%     
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  Files          11       11              
  Lines         136      137       +1     
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+ Hits          105      106       +1     
  Misses         31       31              

Flag	Coverage Δ
docs	74.81% <0.00%> (-0.56%)	⬇️

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[mtfishman]

Looks good, thanks. Can you add a test? Also, I guess we should start listing docstrings in the docs...

[lkdvos]

I tried something out to make a @autodocs style page, this could be something we add to the skeleton once we settle on the design.
The test is a bit silly, but actually did catch that I forgot to add the dependency, so that's nice!