[worklets] Allow lazy construction of WorkletGlobalScope.

[hoch]

In the section 2.2, the step 10 in the algorithm:

If the worklet’s WorkletGlobalScopes is empty, run the following steps:
1. Create a WorkletGlobalScope given workletGlobalScopeType, moduleResponsesMap, 
   and outsideSettings.
2. Add the WorkletGlobalScope to worklet’s WorkletGlobalScopes.

For AudioWorklet, the worklet global scope is associated with BaseAudioContext and it should be created when the context is constructed. The current spec text does not allow that.