Move two-layer shallow water equations from Trixi.jl

[patrickersing]

This PR adds the 1D and 2D version of the ShallowWaterEquationsTwoLayer, that were removed from Trixi.jl in trixi-framework/Trixi.jl#1809

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.52%. Comparing base (4a508f2) to head (8c2b0b3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
+ Coverage   99.25%   99.52%   +0.26%     
==========================================
  Files          33       40       +7     
  Lines         942     1459     +517     
==========================================
+ Hits          935     1452     +517     
  Misses          7        7              

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[patrickersing]

This is basically a direct copy from the files in Trixi.jl and I only had to qualify some of the functions.
I slightly restructured the testsets, which causes most of the deleted lines in this PR.

[andrewwinters5000]

Just a couple questions, but overall this already looks good! It is nice how simple it was to load in a new set of equations.

[andrewwinters5000]

LGTM!