Separate XACT into it's own library Xna.Framework.Audio.Xact

[nkast]

• Xact and it's authoring tool has been discontinued.
• The file format is undocumented and partly reversed engineered.
• The Importer and processor *[1] for .XAP files *[2] was never implemented.

Hardy anyone will ever use Xact for new projects, it's mostly here for porting old XNA games, to the degree that this is possible by the current implementation. We should separate the Xact classes into their own library.

*[1] - https://github.com/kniEngine/XNAGameStudio/blob/751db461c064bede43e867d7ad9848403ef881ab/Samples/Ship-Game/ShipGameContent/ShipGameContentWindows.contentproj#L102-L103
*[2] - https://github.com/kniEngine/XNAGameStudio/blob/main/Samples/Ship-Game/ShipGameContent/sounds/sounds.xap