experiments: template_string_func experiment keyword

This doesn't actually do anything yet, but we'll make it do something in
a future commit.

internal/experiments/experiment.go

@@ -20,6 +20,7 @@ const (
 	VariableValidationCrossRef     = Experiment("variable_validation_crossref")
 	ModuleVariableOptionalAttrs    = Experiment("module_variable_optional_attrs")
 	SuppressProviderSensitiveAttrs = Experiment("provider_sensitive_attrs")
+	TemplateStringFunc             = Experiment("template_string_func")
 	ConfigDrivenMove               = Experiment("config_driven_move")
 	PreconditionsPostconditions    = Experiment("preconditions_postconditions")
 	UnknownInstances               = Experiment("unknown_instances")
@@ -32,6 +33,7 @@ func init() {
 	registerConcludedExperiment(VariableValidation, "Custom variable validation can now be used by default, without enabling an experiment.")
 	registerCurrentExperiment(VariableValidationCrossRef)
 	registerConcludedExperiment(SuppressProviderSensitiveAttrs, "Provider-defined sensitive attributes are now redacted by default, without enabling an experiment.")
+	registerCurrentExperiment(TemplateStringFunc)
 	registerConcludedExperiment(ConfigDrivenMove, "Declarations of moved resource instances using \"moved\" blocks can now be used by default, without enabling an experiment.")
 	registerConcludedExperiment(PreconditionsPostconditions, "Condition blocks can now be used by default, without enabling an experiment.")
 	registerConcludedExperiment(ModuleVariableOptionalAttrs, "The final feature corresponding to this experiment differs from the experimental form and is available in the Terraform language from Terraform v1.3.0 onwards.")