fix: better handling of Enzyme split mode + run Enzyme tests on 1.11

[gdalle]

New attempt at #615.

DI extensions

Enzyme:

• Add EnzymeCore as an explicit dependency of the Enzyme extension.
• Use new mode splitter to forward characteristics of combined mode to its split counterpart

DI tests

Enzyme:

• Remove reverse-over-reverse second-order tests (failing on Julia 1.11 and not very useful)
• Avoid testing matrix inputs on Julia 1.11
  • Forward-over-reverse HVP ignores rows for matrix input, only with Julia 1.11 EnzymeAD/Enzyme.jl#2071
• Skip sumdiffcube and sumdiffcube_mat scenarios on Julia 1.11
  • Error with non-integer sizeof when differentiating Base.diff EnzymeAD/Enzyme.jl#2168
  • Enzyme compilation failed for HVP with broadcast EnzymeAD/Enzyme.jl#2170

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.58%. Comparing base (881fc3f) to head (81e566b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #654      +/-   ##
==========================================
+ Coverage   97.53%   97.58%   +0.05%     
==========================================
  Files         112      111       -1     
  Lines        5561     5552       -9     
==========================================
- Hits         5424     5418       -6     
+ Misses        137      134       -3     

Flag	Coverage Δ
DI	98.74% <100.00%> (+0.07%)	⬆️
DIT	95.36% <ø> (ø)

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