[API Compat] add rule to make sure struct fields follow LayoutKind

[smasher164]

See dotnet/arcade#9892

Structs that follow LayoutKind.Sequential should be flagged if they reorder public fields.
LayoutKind.Explicit will also need to be considered here.

@ericstj @tannergooding

[smasher164]

The compatibility guidelines (https://docs.microsoft.com/en-us/dotnet/core/compatibility/) would also need to be updated to reflect this.

[ericstj]

Yeah, this should be an uncontroversial change 😄 Thanks for filing this, @smasher164

[ViktorHofer]

@carlossanlop if you want to ramp up on APICompat, this would be a great rule to start with. Steps necessary:

1. Add a rule to this folder: https://github.com/dotnet/sdk/tree/main/src/ApiCompat/Microsoft.DotNet.ApiCompatibility/Rules
2. Register the rule in the RuleFactory:

   sdk/src/ApiCompat/Microsoft.DotNet.ApiCompatibility/Rules/RuleFactory.cs

   Lines 30 to 41 in 17661c8

   	List<IRule> rules = new()
   	{
   	new AssemblyIdentityMustMatch(_log, settings, context),
   	new CannotAddAbstractMember(settings, context),
   	new CannotAddMemberToInterface(settings, context),
   	new CannotAddOrRemoveVirtualKeyword(settings, context),
   	new CannotRemoveBaseTypeOrInterface(settings, context),
   	new CannotSealType(settings, context),
   	new EnumsMustMatch(settings, context),
   	new MembersMustExist(settings, context),
   	new CannotChangeVisibility(settings, context)
   	};

3. Update the compatibility guidelines

[tannergooding]

Yeah, this should be an uncontroversial change

@ericstj, @ViktorHofer. The controversial aspect here is going to be that C# defaults to LayoutKind.Sequential if nothing is specified and so all value types will default to it being a breaking change to reorder their fields