Better method of specifying a target source property type for FGD entities

[redxdev]

This is an alternate take on fixing #215 as opposed to using StringName like #240 and I believe sets a better standard for how to handle "extended" property types going forward.

The idea is that to make a target_source property you make a new inline resource of type FuncGodotTargetSourceProperty instead of an Object (since that won't save properly and gets reset to null on reload of the entity definition resource).

A benefit of this approach for future property type support is that additional options could be made available in the resource, such as a proper default value instead of it always being empty.

An alternate approach would be to have a more generic extended property data resource type for specifying custom FGD types like so:

class_name FuncGodotPropertyData
extends Resource

@export var prop_type: String
@export var prop_val: String

but that requires a bit more work.