[JS] Add `fromObject` method to take an AsObject type and return a T.

What language does this apply to?
JS/TS

Describe the problem you are trying to solve.
In JS, many different libraries expect to work with dictionaries, basically Record<A, B> or lists of these, to do things like state management in single page apps, list operations like find, slice, zip, etc. and more. Having the ability to freely move between a Message and a JSON object would make it MUCH easier to interoperate with different tools.

Currently, my message types get compiled to something like the following:

export class Polygon extends jspb.Message {
  // getters and setters here

  serializeBinary(): Uint8Array;
  toObject(includeInstance?: boolean): Polygon.AsObject;
  static toObject(includeInstance: boolean, msg: Polygon): Polygon.AsObject;
  static serializeBinaryToWriter(message: Polygon, writer: jspb.BinaryWriter): void;
  static deserializeBinary(bytes: Uint8Array): Polygon;
  static deserializeBinaryFromReader(message: Polygon, reader: jspb.BinaryReader): Polygon;
}

Describe the solution you'd like

  static fromObject(from: Polygon.AsObject): Polygon;

Describe alternatives you've considered
Writing a generic function to parse the field descriptors, but I don't know enough to make it work correctly, PLUS, it seems like a much better thing to be provided out of the box.

Additional context