SDDE support

[YingboMa]

@variables t x(..)
@parameters a=-4.0 b=-2.0 c=10.0 α=-1.3 β=-1.2 γ=1.1
D = Differential(t)
@brownian η
τ = 1.0
eqs = [D(x(t)) ~ a * x(t) + b * x(t - τ) + c + (α * x(t) + γ) * η]
@named sys = System(eqs)
sys = structural_simplify(sys)
@test equations(sys) == [D(x(t)) ~ a * x(t) + b * x(t - τ) + c]
@test isequal(ModelingToolkit.get_noiseeqs(sys), [α * x(t) + γ;;])
prob_mtk = SDDEProblem(sys, [x(t) => 1.0 + t], tspan; constant_lags = (τ,));
@test_nowarn sol_mtk = solve(prob_mtk, RKMil())

now works.