BridgeJS: Allow extensions to contain types

[wfltaylor]

Currently, you can write this:

@JS enum Change {
    case update(Update)
    case delete

    @JS struct Update {
        let contents: String
    }
}

but not this (which actually emitted broken generated code):

@JS enum Change {
    case update(Update)
    case delete
}

extension Change {
    @JS struct Update {
        let contents: String
    }
}

This PR removes this limitation, so you can define @JS types in extensions.

[wfltaylor]

I’ve pushed a commit to fix an existing .d.ts code-generation issue that the new tests triggered.

[krodak]

Three inline findings. The pre-existing relative qualified-name issue is tracked separately in #805.

[krodak]

FYI, extension scoping now handles unqualified names, but relative qualified names such as Shelf.Divider are still treated as root-qualified. An @JS method using that spelling is omitted from the skeleton instead of resolving it as Library.Shelf.Divider.

This behavior already exists before this PR, so I opened #805 to follow it separately rather than treating it as part of this change.

[krodak]

I'll open up PR for #805 after this one is merged