BridgeJS: Fix issues exporting nested types

[wfltaylor]

There are currently a few issues with the JS/TS code generation, due to a lack of merging. For example, the following Swift code:

@JS struct Control {
    @JS enum Variant: String { case button, slider }

    @JS struct Layout {
        @JS enum Alignment: String { case leading, trailing }
        var padding: Int
    }

    var name: String

    @JS init(name: String) { self.name = name }
}

generates a .d.ts:

export type Exports = {
    Control: {
        init(name: string): Control;
    },
    Control: {
        Variant: VariantObject,
        Layout: {
            Alignment: AlignmentObject
        }
    }
}

This doesn’t type check correctly. Similar issues occur with the emitted .js, except instead of failing to type check, these will cause runtime issues (since keys are overwritten instead of merged).

This PR is a bit of a refactor of BridgeJSLink to address this. Instead of being a member of a NamespaceNode, each class and struct becomes its own NamespaceNode. This makes it much easier to merge everything together so it can be emitted as one unit.