refactor: mark SymbolicContinuousCallback, SymbolicDiscreteCallback as public

Author: AayushSabharwal

src/ModelingToolkit.jl

@@ -338,6 +338,7 @@ export DynamicOptSolution
 @public unbound_outputs, is_bound
 @public AbstractSystem, CheckAll, CheckNone, CheckComponents, CheckUnits
 @public t, D, t_nounits, D_nounits, t_unitful, D_unitful
+@public SymbolicContinuousCallback, SymbolicDiscreteCallback
 
 for prop in [SYS_PROPS; [:continuous_events, :discrete_events]]
     getter = Symbol(:get_, prop)