Figure.choropleth: Refactor using low-level sessions and virtualfiles

[seisman]

Previously, the Figure.choropleth method was implemented by wrapping the Figure.plot method. It's simple, but has the following downsides:

1. The Figure.plot method has complicated codes for dealing with diverse input format, which is necessary for Figure.choropleth
2. Extra parameters are passed to Figure.plot via **kwargs, but many parameters, such as symbol/straight_line/errorbar don't make sense in Figure.choropleth.

This PR reimplements Figure.choropleth using the lower-level plot module call instead of wrapping Figure.plot. Also updates the docstring to describe the method directly instead of as a thin wrapper around Figure.plot.

Strictly speaking, this is a breaking change, as kwargs is removed. Since this method was added after v0.18.0, it's not public yet. So the breaking change should have no effect on existing scripts.

Preview: https://pygmt-dev--4700.org.readthedocs.build/en/4700/api/generated/pygmt.Figure.choropleth.html#pygmt.Figure.choropleth

Address #4414. Supersedes #2798.