[Feature Request] Expose classes TextEncoder and TextDecoder in Workflow sandbox

[mjameswh]

Proposition

Expose WHATWG's TextEncode and TextDecode classes in workflow context.

Why

• This API includes no feature that could break workflow determinism (e.g. no file IO).
• Users have mentioned the need to use this API from a workflow context.
• At the moment, this requires either delegating this usage to a local activity (or a regular activity) or having users load an appropriate polyfill.

[lukeramsden]

This was useful for my usecase (using typeid-js to generate IDs in workflows, the Math.random stub in the isolate is sufficient as these don't need to be cryptographically secure) so I've opened a PR based on a patch I have locally.