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)_.
it("gets document stats after parsing page", async function () {
      await page.getOperatorList();
      const stats = pdfDocument.stats;
      const expectedStreamTypes = {
        [StreamType.FLATE]: true,
      };
      const expectedFontTypes = {
        [FontType.TYPE1STANDARD]: true,
        [FontType.CIDFONTTYPE2]: true,
      };
      expect(stats).toEqual({
        streamTypes: expectedStreamTypes,
        fontTypes: expectedFontTypes,
      });
    });
    it("gets page stats after parsing page, without `pdfBug` set", async function () {
      await page.getOperatorList();
      expect(page.stats).toEqual(null);
    });
    it("gets page stats after parsing page, with `pdfBug` set", async function () {
      const loadingTask = getDocument(
        buildGetDocumentParams(basicApiFileName, { pdfBug: true })
      );
      const pdfDoc = await loadingTask.promise;
      const pdfPage = await pdfDoc.getPage(1);
      await pdfPage.getOperatorList();
      const stats = pdfPage.stats;
      expect(stats instanceof StatTimer).toEqual(true);
      expect(stats.times.length).toEqual(1);
      const [statEntry] = stats.times;