Add MethodImplAttributes.Async flag

[PhilippNaused]

Add the new Async flag for methods that was introduced for the new Runtime-Async Feature in .NET 11.

See:
https://github.com/dotnet/runtime/blob/main/docs/design/specs/runtime-async.md

https://github.com/dotnet/runtime/blob/0c7ef81aa7d25dcac00eb81d156e03f4c0ca396b/src/coreclr/inc/corhdr.h#L650

https://github.com/dotnet/runtime/blob/0c7ef81aa7d25dcac00eb81d156e03f4c0ca396b/src/libraries/System.Private.CoreLib/src/System/Reflection/MethodImplAttributes.cs#L34

[ElektroKill]

MethodDef.HasReturnType could also potentially be updated to match this statement:

"Async methods also do not have matching return type conventions as sync methods. For sync methods, the stack should contain a value convertible to the stated return type before the ret instruction. For async methods, the stack should be empty in the case of Task or ValueTask, or the type argument in the case of Task<T> or ValueTask<T>."

found in https://github.com/dotnet/runtime/blob/main/docs/design/specs/runtime-async.md#i845-sync-and-async-methods

@wtfsck What do you think?

[PhilippNaused]

@ElektroKill Like this?

 /// <summary>
 /// <c>true</c> if the method returns a value (i.e., return type is not <see cref="System.Void"/>)
 /// </summary>
+/// <remarks>
+/// <see cref="MethodImplAttributes.Async"/> methods also do not have matching return type conventions as sync methods.
+/// For sync methods, the stack should contain a value convertible to the stated return type before the <see cref="OpCodes.Ret"/> instruction.
+/// For async methods, the stack should be empty in the case of <c>Task</c> or <c>ValueTask</c>, or the type argument in the case of <c>Task&lt;T&gt;</c> or <c>ValueTask&lt;T&gt;</c>.
+/// <seealso href="https://github.com/dotnet/runtime/blob/main/docs/design/specs/runtime-async.md"/>
+/// </remarks>
 public bool HasReturnType => ReturnType.RemovePinnedAndModifiers().GetElementType() != ElementType.Void;