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_esp8266.S

@@ -18,7 +18,7 @@ tinygo_startTask:
     callx0 a12
 
     // After return, exit this goroutine. This is a tail call.
-    call0  tinygo_pause
+    call0  tinygo_task_exit
 .size tinygo_startTask, .-tinygo_startTask
 
 .global tinygo_swapTask