feat: add least-squares linear solver

[gdalle]

• Define IterativeLeastSquaresSolver for situations where the conditions $c$ have a different type than the solution $y$
• In these cases, we need both $A$ and $A^\top$ (as linear operators or matrices), which required updating the linear solver interface
• Add tests and fix the default solver so that $A \neq A^\top$

Codecov is not updating so I'm gonna merge to see what it looks like on main

[codecov]

Codecov Report

❌ Patch coverage is 69.44444% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/settings.jl	50.00%	7 Missing ⚠️
ext/ImplicitDifferentiationChainRulesCoreExt.jl	76.92%	3 Missing ⚠️
ext/ImplicitDifferentiationForwardDiffExt.jl	85.71%	1 Missing ⚠️

Files with missing lines	Coverage Δ
ext/ImplicitDifferentiationZygoteExt.jl	100.00% <100.00%> (ø)
src/ImplicitDifferentiation.jl	100.00% <ø> (ø)
src/implicit_function.jl	100.00% <ø> (ø)
ext/ImplicitDifferentiationForwardDiffExt.jl	95.45% <85.71%> (-4.55%)	⬇️
ext/ImplicitDifferentiationChainRulesCoreExt.jl	89.28% <76.92%> (-6.37%)	⬇️
src/settings.jl	74.07% <50.00%> (-25.93%)	⬇️

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