Correct declaration emit of commonjs exports of namespace-nested classes

[sandersn]

Followup to #40228:

var NS = {}
NS.K =class {
    values() {
        return new NS.K()
    }
}
exports.K = NS.K;

Expected behavior:

Should produce d.ts output:

declare namespace NS {
    class _K {
      values(): _K;
    }
    export {_K as K};
}
import _K = NS.K;
export {_K as K};

Actual behavior:

export var K: {
    new (): {
        values(): any;
    };
};

1. Inlines and destroys the namespace.
2. Only exports a value, not a class.
3. Doesn't serialise nested typed references to K properly.

The declaration emitter just needs code to support this; right now it's hitting inaccurate fallback code.