LibWeb: Implement CanvasRenderingContext2D.isContextLost()

Note that we don't implement the "context lost steps" yet, so this will
always return the initial value (false).

Userland/Libraries/LibWeb/HTML/CanvasRenderingContext2D.idl

@@ -34,6 +34,7 @@ interface CanvasRenderingContext2D {
     undefined save();
     undefined restore();
     undefined reset();
+    boolean isContextLost();
 
     readonly attribute HTMLCanvasElement canvas;