populate remaining missing return types, using native returns where possible

this is done separately to stable so that stable changes can be integrated easily using an empty merge.

src/world/format/SubChunk.php

@@ -111,7 +111,7 @@ class SubChunk implements SubChunkInterface{
 		$this->blockLight = $data;
 	}
 
-	public function __debugInfo(){
+	public function __debugInfo() : array{
 		return [];
 	}