feat: support custom HTML/SVG element markers in MapLibre

Add Element, Anchor, and Rotation fields to the Marker struct so users
can render custom icons (ships, circles, buildings) instead of the
default pin. When Element is set, Color is ignored and the raw HTML/SVG
is injected as the marker's DOM element.

Update the example to demonstrate a static ship SVG marker with rotation
and a signal-driven vehicle marker using a custom SVG circle.

maplibre/types.go

@@ -302,6 +302,19 @@ type Marker struct {
 	Draggable bool
 	Popup     *Popup
 
+	// Element is raw HTML/SVG used as a custom marker instead of the
+	// default pin. When set, Color is ignored.
+	// Do not pass untrusted user input without sanitizing it first.
+	Element string
+
+	// Anchor controls which part of the element sits at the coordinate.
+	// Values: "center" (default for custom elements), "bottom" (default
+	// for the pin), "top", "left", "right", "top-left", etc.
+	Anchor string
+
+	// Rotation is clockwise degrees. Useful for directional icons (ships, vehicles).
+	Rotation float64
+
 	// Signal-backed position. When set, signals drive marker position reactively.
 	// Initial position is read from the signal values. LngLat is ignored when signals are set.
 	// If Draggable is true, drag updates write back to these signals.