Move coord projection selection onto coord classes

[iangow]

Upstream PR review item from has2k1#1059.

Source review comments:

• Added two new coordinate systems: coord_polar and coord_radial. has2k1/plotnine#1059 (comment)
• Added two new coordinate systems: coord_polar and coord_radial. has2k1/plotnine#1059 (comment)

Requested change:

• Add a projection property to the base coord class, likely _projection: str | None = None.
• Set _projection = "polar" on coord_polar and any radial/polar subclasses that need Matplotlib polar axes.
• Update facet subplot creation to use projection=self.plot.coordinates._projection instead of special-casing coordinate classes in facet.py.

Review goal: limit the external footprint of custom coord classes outside their own modules.

[iangow]

Addressed by commit 04a508c: Move projection selection to coord classes