Explore extensible themeables for custom coord classes

[iangow]

Follow-up idea from the upstream PR review thread in has2k1#1059.

Source comment:

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

Context:

• Built-in coord classes can move theme-specific adjustments into plotnine's existing themeables.
• External custom coord classes cannot currently modify plotnine's internal themeables directly.
• The reviewer noted that custom coord classes could do some manipulation in coord.setup_ax, and that making themeables extendable may be worth considering later.

Potential work:

• Investigate whether plotnine should expose an extension mechanism for third-party themeables.
• Decide whether this should be a documented API, internal hook, or deferred until a concrete custom-coord use case needs it.

Priority: lower than the immediate PR refactors.

[iangow]

Addressed by commit bbfb072: Document custom themeable extension path