Worklet: Separate Worklet into MainThreadWorklet and ThreadedWorklet
This is a preparation patch for introducing module script loading for main
thread worklets. This should not change behavior.
In the current implementation, classic script loading code is tied with Worklet
class that is the common class of main thread worklets and threaded worklets.
This is an obstacle to introduce module script loading only in main thread
This CL separates the Worklet class into MainThreadWorklet and ThreadedWorklet
as a stopgap. MainThreadWorklet will implement module script loading in
following patches and ThreadedWorklet will keep classic script loading until
module script loading becomes available for ThreadedWorklets.