Provide a reliable way to see original resources in a directory

When exporting a project, resources are often moved, converted or
remapped (source import files are gone, text scenes are converted to binary,
etc).

This new function allows to list a directory and obtain the actual original
resource files.

Example

```GDScript
var resources = ResourceLoader.list_directory("res://images")
print(resources)
```
Result will be something like:
```
["image1.png","image2.png","image3.png"]
```
instead of
```
["image1.png.import","image2.png.import","image3.png.import"]
```

core/core_bind.h

@@ -87,6 +87,8 @@ public:
 	bool exists(const String &p_path, const String &p_type_hint = "");
 	ResourceUID::ID get_resource_uid(const String &p_path);
 
+	Vector<String> list_directory(const String &p_directory);
+
 	ResourceLoader() { singleton = this; }
 };