feat: allow creating an image source without a url (#8167)

* feat: allow creating an image source without a url

* chore(deps): bump @maplibre/maplibre-gl-style-spec to 26.4.0

Makes the image source's url optional in both the TypeScript type and the
v8 runtime spec, which serialize() and addSource validation need.

* chore: adapt icon-rotation-alignment to its data-driven spec type