Add a function similar to SkyCoord.directional_offset_by to PixCoord

A function similar to [SkyCoord.directional_offset_by](https://docs.astropy.org/en/stable/api/astropy.coordinates.SkyCoord.html#astropy.coordinates.SkyCoord.directional_offset_by) could be added to PixCoord. This will help support the DS9 vector region, which takes x, y, separation, and angle as parameters.