Factory macro

A macro that produces a factory method for classes.

Example:

```
fac class_name uid
fac iCardSelectionEntry uid://5jf8ycnetcx7
```

```GDScript
static func make_instance() -> iCardSelectionEntry:
	var i: iCardSelectionEntry = load("uid://5jf8ycnetcx7").instantiate()
	return i
```