chore: export subset_tunables

Author: DhairyaLGandhi

docs/src/API/model_building.md

@@ -226,7 +226,7 @@ change_independent_variable
 add_accumulations
 noise_to_brownians
 convert_system_indepvar
-ModelingToolkit.subset_tunables
+subset_tunables
 ```
 
 ## Hybrid systems

src/ModelingToolkit.jl

@@ -265,7 +265,8 @@ export isinput, isoutput, getbounds, hasbounds, getguess, hasguess, isdisturbanc
        istunable, getdist, hasdist,
        tunable_parameters, isirreducible, getdescription, hasdescription,
        hasunit, getunit, hasconnect, getconnect,
-       hasmisc, getmisc, state_priority
+       hasmisc, getmisc, state_priority,
+       subset_tunables
 export liouville_transform, change_independent_variable, substitute_component,
        add_accumulations, noise_to_brownians, Girsanov_transform, change_of_variables
 export PDESystem
@@ -345,6 +346,7 @@ export DynamicOptSolution
 @public collect_var_to_name!, collect_vars!, eqtype_supports_collect_vars, hasdefault
 @public getdefault, setdefault, iscomplete, isparameter, modified_unknowns!
 @public renamespace, namespace_equations
+@public subset_tunables
 
 for prop in [SYS_PROPS; [:continuous_events, :discrete_events]]
     getter = Symbol(:get_, prop)