Strongly type Resource .tres files in ResourceTree when possible

[Seb105]

Resource files can contain type information when they are created from a script which is a GlobalClass/Tool

This PR makes [ResourceTree] search for this information, and where it can, load the resource as that type.

As an example:

[GlobalClass, Tool]
public partial class InventoryItemSet : Resource 
{
    // ...
}

Produces this header:

[gd_resource type="Resource" script_class="InventoryItemSet" format=3 uid="uid://cfpe2cbwdix4h"]

This PR makes the ResourceTree source generator look for script_class="Type" in types that are found to be a Resource, and use that type where possible.

Where script_class is not present or no valid type is found, it is simply treated as Resource.