Map: Add style variations

[iandunn]

#504 introduced a last-minute change of the marker icon color. In the near future we'll need to support different icon colors on different pages, and also different styles. We should revert that PR and refactor to a more proper long-term solution. We shouldn't have to add new SVG files just to change the color. There should be block attributes to change the color, size, etc.

We may also want to support different images, but that is probably for another issue when/if we actually need it.

Inserting the image into the DOM as <svg> instead of <img> should allow using CSS fill. See https://developers.google.com/maps/documentation/javascript/examples/marker-modern for a possible way to do that, but I haven't looked closely.

We'll also need to support different map tile colors, etc. It's probably best to create "themes" as style variations. The existing WP20 could be the default, then there could be a variation for SotW 2023 which has the red icons, silver map BG, etc.