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"]
```

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