make eigvals(::Diagonal) accept sortby

[araujoms]

Solves the Diagonal part of #1471 and #1450.

I thought it would be better to start with Diagonal, as it was the only part that required a bit of thinking. I think it also makes sense in itself, since eigen accepted sortby, just eigvals and eigvecs did not. Even though the docstring said none did.

If this is welcome I'll also do SymTridiagonal, and then we can document that eigenvalues are sorted by default.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.91%. Comparing base (7e11b5e) to head (d1f41eb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1477      +/-   ##
==========================================
+ Coverage   93.89%   93.91%   +0.01%     
==========================================
  Files          34       34              
  Lines       15933    15934       +1     
==========================================
+ Hits        14961    14965       +4     
+ Misses        972      969       -3     

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