refactor: mark collect_* functions as public

Author: AayushSabharwal

docs/src/API/System.md

@@ -200,3 +200,15 @@ ModelingToolkit.CheckUnits
 ```
 
 These can also be used by custom `AbstractSystem` subtypes.
+
+## Utility functions
+
+These utility functions can be useful when manipulating systems, especially when building
+custom `AbstractSystem` subtypes.
+
+```@docs
+ModelingToolkit.collect_scoped_vars!
+ModelingToolkit.collect_var_to_name!
+ModelingToolkit.collect_vars!
+ModelingToolkit.eqtype_supports_collect_vars
+```

src/ModelingToolkit.jl

@@ -341,7 +341,8 @@ export DynamicOptSolution
 @public SymbolicContinuousCallback, SymbolicDiscreteCallback
 @public VariableType, MTKVariableTypeCtx, VariableBounds, VariableConnectType
 @public VariableDescription, VariableInput, VariableIrreducible, VariableMisc
-@public VariableOutput, VariableStatePriority, VariableUnit
+@public VariableOutput, VariableStatePriority, VariableUnit, collect_scoped_vars!
+@public collect_var_to_name!, collect_vars!, eqtype_supports_collect_vars
 
 for prop in [SYS_PROPS; [:continuous_events, :discrete_events]]
     getter = Symbol(:get_, prop)