Add stroke_dash_array option to TimeSliderChoropleth

[ChrisJr404]

Follow-up to #1615. That issue asked to make the stroke around TimeSliderChoropleth features configurable, and #1838 added stroke_color, stroke_width and stroke_opacity. But the dash pattern is still hard-coded in the template:

.attr('stroke-dasharray', '5,5')

So the white dashed line the original reporter wanted to get rid of can be recolored or hidden, but you still can't make it solid. This adds a stroke_dash_array argument that feeds straight into the SVG stroke-dasharray:

TimeSliderChoropleth(data, styledict, stroke_dash_array="0")  # solid line

The default is "5,5", which renders exactly like before, so existing maps are unchanged. I also documented the three earlier stroke options in the docstring while I was there, since they weren't listed yet.

Testing

Two new tests in tests/plugins/test_time_slider_choropleth.py: one checks the default still emits stroke-dasharray', '5,5', the other that stroke_dash_array="0" emits '0' and no longer the dashed default.

pytest tests/plugins/test_time_slider_choropleth.py is 3 passed; the only failure is test_timedynamic_geo_json, which needs geodatasets/geopandas and fails the same way on an unmodified checkout. ruff, black and codespell are clean on both files.