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 hasJSActions = await pdfDoc.hasJSActions();
      expect(hasJSActions).toEqual(true);
      await loadingTask.destroy();
    });
    it("gets non-existent JSActions", async function () {
      const jsActions = await pdfDocument.getJSActions();
      expect(jsActions).toEqual(null);
    });
    it("gets JSActions", async function () {
      
      const loadingTask = getDocument(
        buildGetDocumentParams("doc_actions.pdf")
      );
      const pdfDoc = await loadingTask.promise;
      const docActions = await pdfDoc.getJSActions();
      const page1 = await pdfDoc.getPage(1);
      const page1Actions = await page1.getJSActions();
      const page3 = await pdfDoc.getPage(3);
      const page3Actions = await page3.getJSActions();
      expect(docActions).toEqual({
        DidPrint: [`this.getField("Text2").value = "DidPrint";`],
        DidSave: [`this.getField("Text2").value = "DidSave";`],
        WillClose: [`this.getField("Text1").value = "WillClose";`],
        WillPrint: [`this.getField("Text1").value = "WillPrint";`],
        WillSave: [`this.getField("Text1").value = "WillSave";`],
      });
      expect(page1Actions).toEqual({
        PageOpen: [`this.getField("Text1").value = "PageOpen 1";`],
        PageClose: [`this.getField("Text2").value = "PageClose 1";`],