Suppress reporting VSTHRD114 diagnostic on Task? returning methods

[bluetarpmedia]

VSTHRD114 now ignores C# methods returning Task?

Converted VSTHRD114 to an abstract base class in order to use LanguageUtils.

This implementation uses syntax nodes to detect the nullable return type, which means the Roslyn CodeAnalysis does not need to be upgraded.

It prevents VSTHRD114 warnings on cases like these:

public Task? GetTask()
{
    return null;
}

public Task<int>? GetTaskOfInt()
{
    return null;
}

public Task<object?>? GetTaskOfNullableObject()
{
    return null;
}

public void LocalFunc()
{
    Task<object>? GetTaskObj()
    {
        return null;
    }
}

But it doesn't prevent warnings for anonymous functions or lambdas, like:

public void StillProducesWarning()
{
    Func<Task?> func = () =>
    {
        return null;
    };
}

If and when the Roslyn upgrade happens the implementation can be simplified to use IMethodSymbol.ReturnNullableAnnotation and that will solve the anon/lambda cases too.

Closes #637

[AArnott]

Very well done. Thank you!