Move the core of OpenRA.Mods.RA.Scripting to OpenRA.Mods.Common.Scripting

Depends on #6750 for moving `Scripting\Properties\UpgradeProperties.cs`.
We didn't really enforce that rule in the Lua API files (for good reason, I think), but rather collected related code in the same file (see TransportProperties.cs, PlaneProperties.cs, CoordinateGlobal.cs etc. for examples). And I absolutely don't want a 1:1 mapping of traits to Lua API source files (CaptureProperties.cs, ExternalCaptureProperties.cs... madness!).

But oh well, we can always change it later.