feat: add connection graph utilities

Author: AayushSabharwal

src/ModelingToolkit.jl

@@ -161,6 +161,7 @@ include("systems/index_cache.jl")
 include("systems/parameter_buffer.jl")
 include("systems/abstractsystem.jl")
 include("systems/model_parsing.jl")
+include("systems/connectiongraph.jl")
 include("systems/connectors.jl")
 include("systems/state_machines.jl")
 include("systems/analysis_points.jl")