POC: Add the Position class for GMT embellishment placement

[seisman]

See #4168 (comment) for context.

This PR implements the Position class.

Usage:

>>> import pygmt
>>> fig = pygmt.Figure()
>>> fig.basemap(region=[0, 10, 0, 10], projection="X10c", frame=True)
>>> fig.logo(
...:     position=pygmt.params.Position((3, 3), type="mapcoords", anchor="ML", offset=(0.2, 0.2)),
...:     box=True,
...: )
>>> fig.show()