InitiatorType for module scripts

[yoavweiss]

It seems like the initiatorType for module script preloads is currently tested to be "other"

Would be good to define it to be either more meaningful ("script" or "module script") or more consistent ("link" like other <link> based resources)

[clelland]

The spec currently says that this should be "module"
If switching to something like "module script", then this needs to be defined in HTML first, and aligned with the list in fetch as well.

[yoavweiss]

HTML's set up the module script request doesn't set an initiatorType on the request. We should modify it to indicate "script".