Export the coordinate-system and domain plots as Python

### Task Summary

Eighteen more visualizations implement `StandaloneCodeGenerator`, the ones that draw in a space of their own rather than over a table's columns: the polar family (polar, radar in its two forms, wind rose), the ternary pair, the three-dimensional scatter, the contour and carpet plots, the quiver field, the choropleth map, the gantt chart and time series over a clock, the gauge and bullet indicators, the dumbbell and filled-area comparisons, and the volcano plot. With these and the structure charts, every visualization Texera offers can be exported.

Each is compared as a figure rather than as a frame, after both paths have drawn. A plot's meaning is in the numbers behind it, so that is what the comparison reads; comparing the rendered picture instead would fail over a layout detail that carries no meaning.

Two behaviour fixes travel with these plots. The Filled Area Plot requires its line group once the switch that reads it is on, and the Contour Plot says what it cannot contour instead of ending the run.

Step 16 of 27 in #8325. It needs #8327 for the trait. Those two behaviour fixes are filed separately as #8283 and #8076 and close with this work.