How to use the "Run Swift Script" command?

For future reference to anyone else who might go down a similar rabbit hole, the best compromise I could find/make was this:

public protocol iStat
{
	var _Value: Int64 {get set}
	var Value: Int64 {get}
	mutating func SetValue(_ _new_value: Int64)
}

extension iStat
{
	public var Value: Int64 {get {return _Value}}
	
	public mutating func SetValue(_ _new_value: Int64)
	{
		_Value = _new_value
	}
}

public class InternalTest : iStat
{
	@_spi(internal_only) public var _Value: Int64 = 0

	public init(){}
	
}

Alex’s method above also works fine as well, but if you want to achieve this through an extension, this is the only way to do it AFAIK.