Process: split getMemoryUsage() into 2 functions

this isn't released yet so it's OK to change.
phpstan level 7 doesn't like these kinds of ambiguous return types because there's no way for it to tell which type is returned without a return type specifying extension, and it's easier to just change the API than to make PHPStan understand it.

src/pocketmine/scheduler/SendUsageTask.php

@@ -136,7 +136,7 @@ class SendUsageTask extends AsyncTask{
 					"historyList" => array_values($playerList)
 				];
 
-				$info = Process::getMemoryUsage(true);
+				$info = Process::getAdvancedMemoryUsage();
 				$data["system"] = [
 					"mainMemory" => $info[0],
 					"totalMemory" => $info[1],