[worklets] "destination" of script fetching should be overridden by each worklet spec

[nhiroki]

The "fetch a worklet script" algorithm specifies "script" as a destination.

1. Fetch a module worker script graph given moduleURLRecord, outsideSettings, "script", credentialOptions, and insideSettings.

This should be overridden by each worklet spec (e.g., "paintworklet" for PaintWorklet), but there is no way to do it.

cc: @annevk @domenic

[nhiroki]

I just noticed this issue is the same with #378