Add IBaseWidget.serialize flag (#915)

https://github.com/Comfy-Org/ComfyUI_frontend/pull/3323

Some widgets are only used for display purpose (The preview image
widget), their value shouldn't be serialized into the workflow. This PR
adds a flag to allow widget to skip value serialization.

src/LGraphNode.ts

@@ -777,6 +777,7 @@ export class LGraphNode implements Positionable, IPinnable, IColorable {
     if (widgets && this.serialize_widgets) {
       o.widgets_values = []
       for (const [i, widget] of widgets.entries()) {
+        if (widget.serialize === false) continue
         // @ts-expect-error #595 No-null
         o.widgets_values[i] = widget ? widget.value : null
       }