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 pdfDoc = await loadingTask.promise;
      const pdfPage = await pdfDoc.getPage(1);
      const tree = await pdfPage.getStructTree();
      expect(tree).toEqual({
        role: "Root",
        children: [
          {
            role: "Document",
            lang: "en-US",
            children: [
              {
                role: "H1",
                children: [
                  {
                    role: "NonStruct",
                    children: [{ type: "content", id: "page2R_mcid0" }],
                  
                ],
              
              {
                role: "P",
                children: [
                  {
                    role: "NonStruct",
                    children: [{ type: "content", id: "page2R_mcid1" }],
                  
                ],
              
              {
                role: "H2",