Stopwatch class with ability to add existing value to elapsed time

[Alexanderius]

Specification:

public OffsetStopwatch(TimeSpan startOffset = default)
{
	StartOffset = startOffset;
}

public TimeSpan StartOffset { get; }

public TimeSpan Elapsed => throw new NotImplementedException();
public long ElapsedMilliseconds => throw new NotImplementedException();
public long ElapsedTicks => throw new NotImplementedException();

public new static OffsetStopwatch StartNew()
{
	throw new NotImplementedException();
}