LibWeb: Add CanvasRenderingContext2D.rotate()

This is pretty limited since we don't have wholesale mapping through
the context transform, but we have to start somewhere. :^)

Libraries/LibWeb/DOM/CanvasRenderingContext2D.idl

@@ -5,6 +5,7 @@ interface CanvasRenderingContext2D {
 
     void scale(double x, double y);
     void translate(double x, double y);
+    void rotate(double radians);
 
     void beginPath();
     void closePath();