experiments: variable_validation_crossref is a current experiment

As of this commit this experiment doesn't actually change any behavior at
all; as usual, those changes will follow in subsequent commits.

internal/experiments/experiment.go

@@ -17,6 +17,7 @@ type Experiment string
 // identifier so that it can be specified in configuration.
 const (
 	VariableValidation             = Experiment("variable_validation")
+	VariableValidationCrossRef     = Experiment("variable_validation_crossref")
 	ModuleVariableOptionalAttrs    = Experiment("module_variable_optional_attrs")
 	SuppressProviderSensitiveAttrs = Experiment("provider_sensitive_attrs")
 	ConfigDrivenMove               = Experiment("config_driven_move")
@@ -29,6 +30,7 @@ func init() {
 	// a current or a concluded experiment.
 	registerConcludedExperiment(UnknownInstances, "Unknown instances are being rolled into a larger feature for deferring unready resources and modules.")
 	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.")
 	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.")