feat(experiments): add SCOPED_INCLUDES experiment

Add new experiment flag TASK_X_SCOPED_INCLUDES for scoped variable
resolution in included Taskfiles. When enabled, variables from included
Taskfiles will be isolated rather than merged globally.

This is the first step towards implementing lazy DAG-based variable
resolution with strict isolation between includes.

experiments/experiments.go

@@ -19,6 +19,7 @@ var (
 	GentleForce     Experiment
 	RemoteTaskfiles Experiment
 	EnvPrecedence   Experiment
+	ScopedIncludes  Experiment
 )
 
 // Inactive experiments. These are experiments that cannot be enabled, but are
@@ -43,6 +44,7 @@ func ParseWithConfig(dir string, config *ast.TaskRC) {
 	GentleForce = New("GENTLE_FORCE", config, 1)
 	RemoteTaskfiles = New("REMOTE_TASKFILES", config, 1)
 	EnvPrecedence = New("ENV_PRECEDENCE", config, 1)
+	ScopedIncludes = New("SCOPED_INCLUDES", config, 1)
 	AnyVariables = New("ANY_VARIABLES", config)
 	MapVariables = New("MAP_VARIABLES", config)
 }