Provide a ChunkUtils stub for PHPStan, so it doesn't get confused when the extension is loaded

for some reason the presence of the type problems were reported as
problems on ChunkUtils.php, despite being outside of the project scope
and the errors not applying to the polyfill.

phpstan.neon.dist

@@ -20,6 +20,7 @@ parameters:
 		- pocketmine\DEBUG
 	stubFiles:
 		- tests/phpstan/stubs/pthreads.stub
+		- tests/phpstan/stubs/chunkutils.stub
 	reportUnmatchedIgnoredErrors: false #no other way to silence platform-specific non-warnings
 	ignoreErrors:
 		-

tests/phpstan/stubs/chunkutils.stub

@@ -0,0 +1,26 @@
+<?php
+
+namespace pocketmine\level\format\io;
+
+class ChunkUtils{
+	/**
+	 * @param string $byte_array
+	 *
+	 * @return string
+	 */
+	public static function reorderByteArray($byte_array){}
+
+	/**
+	 * @param string $nibble_array
+	 *
+	 * @return string
+	 */
+	public static function reorderNibbleArray($nibble_array){}
+
+	/**
+	 * @param int[] $color_array
+	 *
+	 * @return string
+	 */
+	public static function convertBiomeColors($color_array){}
+}