I would recommend to use https://www.jetbrains.com/phpstorm/ this instead of visual studio code because it will look better and be a lot easier to implement pocketmine with it. If you're worried about the trial on your jetbrains product ending, either make another account and use that trial. Or apply for a github student developer pack and use the jetbrains key from that. https://education.github.com/pack (all you need is a school email to apply for it. Personally I don't know how to use the pocketmine library with visual studio code. You could download the source code and make a folder called pocketmine with all the files in here https://github.com/pmmp/PocketMine-MP/tree/stable/src in it.