Add NullableExtensions.DangerousGetValueOrNullReference

[Sergio0694]

Closes #61

This PR adds the NullableExtensions.DangerousGetValueOrNullReference method.
It also adjusts the namespace for NullableExtensions, which was incorrect.

cc. @rickbrew

API breakdown:

namespace CommunityToolkit.HighPerformance
{
    public static class NullableExtensions
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static unsafe ref T DangerousGetValueOrNullReference<T>(ref this T? value)
            where T : struct;
    }
}

[rickbrew]

lgtm 😺👍🏼