feat: allow using System as a type

Author: AayushSabharwal

src/systems/systems.jl

@@ -1,3 +1,5 @@
+const System = AbstractODESystem
+
 function System(eqs::AbstractVector{<:Equation}, iv, args...; name = nothing,
         kw...)
     ODESystem(eqs, iv, args...; name, kw..., checks = false)

test/odesystem.jl

@@ -1806,3 +1806,9 @@ end
     @test ModelingToolkit.get_metadata(sys) == "TEST"
     @test ModelingToolkit.get_metadata(sys2) == "TEST"
 end
+
+@testset "`System` works as a type" begin
+    @variables x(t)
+    @named sys = System([D(x) ~ 2x], t)
+    @test sys isa System
+end