Add Source Generator which adds virtual interface methods to C#

[YoshiRulz]

Sorta like C# 8 default interface methods, but the base implementations are filled in at compile-time. Applying the attribute like this:

BizHawk/src/BizHawk.Emulation.Common/Interfaces/Services/IDebuggable.cs

Lines 42 to 43 in 959cb29

	[VirtualMethod]
	bool CanStep(StepType type);

generates

namespace BizHawk.Emulation.Cores.Atari.Atari2600
{
	public partial class Atari2600
	{
		bool BizHawk.Emulation.Common.IDebuggable.CanStep(BizHawk.Emulation.Common.StepType type)
			=> BizHawk.Emulation.Common.IDebuggable.MethodDefaultImpls.CanStep(this, type);
	}
}

unless the base implementation is overridden in that class. The base implementation being this:

BizHawk/src/BizHawk.Emulation.Common/Interfaces/Services/IDebuggable.cs

Lines 15 to 21 in 959cb29

	public interface IDebuggable : IEmulatorService
	{
	public static class MethodDefaultImpls
	{
	public static bool CanStep(IDebuggable self, StepType type)
	=> false;
	}

TODO:

• warning for attr used on member of class/struct/record?
• warning for only one of get/set/add/remove pair has attr?
• warning for unused base implementation (i.e. impl/override exists in every direct implementor)?
• let an interface override a superinterface's virtual method
• find more places this would be helpful
  • IStateable.AvoidRewind
• (out of scope) reflectionless services https://gitlab.com/TASVideos/BizHawk/-/snippets/2010501