canvas2d: Implement .reset() (#31258)

* Implement Canvas2D reset

* Update WPT tests

* Apply suggestions from code review

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Co-authored-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/webidls/CanvasRenderingContext2D.webidl

@@ -44,6 +44,7 @@ interface mixin CanvasState {
   // state
   undefined save(); // push state on state stack
   undefined restore(); // pop state stack and restore state
+  undefined reset();
 };
 
 interface mixin CanvasTransform {