Add support for outline items, in the default viewer, which default to collapsed when the outline is built

The PDF specification supports this feature, which is commonly used in large/long documents (such as the spec itself), and it seems reasonably straightforward to implement; see https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#G11.2095911

src/display/api.js

@@ -715,6 +715,7 @@ class PDFDocumentProxy {
    *     bold: boolean,
    *     italic: boolean,
    *     color: rgb Uint8ClampedArray,
+   *     count: integer or undefined,
    *     dest: dest obj,
    *     url: string,
    *     items: array of more items like this