internal/task: rename tinygo_pause to tinygo_task_exit

This is more descriptive: the call is to exit a task, not to pause it.
This also makes it more obvious that there's an optimization
opportunity: to free the stack explicitly after the goroutine returns
(or to keep it as a cache for the next stack allocation).

src/internal/task/task_stack.go

@@ -54,8 +54,9 @@ func Pause() {
 	currentTask.state.pause()
 }
 
-//export tinygo_pause
-func pause() {
+//export tinygo_task_exit
+func taskExit() {
+	// TODO: explicitly free the stack after switching back to the scheduler.
 	Pause()
 }