feat: make deprecated ODESystem alias System type

Author: AayushSabharwal

src/deprecations.jl

@@ -9,6 +9,8 @@ macro mtkbuild(exprs...)
     end |> esc
 end
 
+const ODESystem = System
+
 for T in [:ODESystem, :NonlinearSystem, :DiscreteSystem, :ImplicitDiscreteSystem]
     @eval @deprecate $T(args...; kwargs...) System(args...; kwargs...)
 end