Config: Properly prevent keys getting transformed into bools

The original regex almost completely failed at its objective, because it a) only worked if there was no value for the key, and b) did not prevent all such occurrences getting transformed, while quoting patterns that would not get transformed anyway.

src/pocketmine/utils/Config.php

@@ -111,7 +111,7 @@ class Config{
 	 * @return string
 	 */
 	public static function fixYAMLIndexes(string $str) : string{
-		return preg_replace("#^([ ]*)([a-zA-Z_]{1}[ ]*)\\:$#m", "$1\"$2\":", $str);
+		return preg_replace("#^( *)(y|Y|yes|Yes|YES|n|N|no|No|NO|true|True|TRUE|false|False|FALSE|on|On|ON|off|Off|OFF)( *)\:#m", "$1\"$2\"$3:", $str);
 	}
 
 	/**