Hello!

Let me know if i should have opened this issue on the [pdfjs-dist](https://github.com/mozilla/pdfjs-dist) repo but I though here would fit.

I'm using pdfjs-dist and I'm in need of some more types to be exported. I have no idea how the types end up in the dist-repo but would love to open a PR if only someone could point me in the right direction 😃 Since I was using an older version of pdfjs-dist I realize the only type I need that's not in the type defs in the latest release are _PageViewport_ from _[display/display_utils](https://github.com/mozilla/pdf.js/blob/master/src/display/display_utils.js#L153)_.
const [{ PDFNetworkStream }, { PDFFetchStream }] = await streamsPromise;
    if (isValidFetchUrl(params.url)) {
      return new PDFFetchStream(params);
    }
    return new PDFNetworkStream(params);
  });
} else if (PDFJSDev.test("GENERIC || CHROME")) {
  if (PDFJSDev.test("GENERIC") && isNodeJS) {
    const { PDFNodeStream } = require("./display/node_stream.js");
    setPDFNetworkStreamFactory(params => {
      return new PDFNodeStream(params);
    });
  } else {
    const { PDFNetworkStream } = require("./display/network.js");
    const { PDFFetchStream } = require("./display/fetch_stream.js");
    setPDFNetworkStreamFactory(params => {
      if (isValidFetchUrl(params.url)) {
        return new PDFFetchStream(params);
      }
      return new PDFNetworkStream(params);
    });
  }
}
export {
  AnnotationLayer,
  AnnotationMode,
  build,
  CMapCompressionType,
  createObjectURL,
  createPromiseCapability,