[worklets-1] should require MIME type enforcement for worklet-loaded script

[dbaron]

There's some effort (/cc @mikewest) to move towards only executing script when it has a correct script MIME Type. This is very useful for a bunch of security reasons. This requires quite an effort to deal with backwards compatibility.

However, where we don't have to worry much about backwards compatibility, like worklets, it seems like we should require this off-the-bat so that we don't have to do it with backwards-compatibility pain later.

[mikewest]

Worklets look like they layer on top of the module fetching code in HTML, which does the MIME type checking in step 8 of https://html.spec.whatwg.org/multipage/webappapis.html#fetch-a-single-module-script. I think we're fine here. :)

[dbaron]

Yes, agreed. Should have looked more closely before filing. I did file whatwg/html#3854 as well, though.