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/core/obj.js

@@ -144,6 +144,7 @@ class Catalog {
       const title = outlineDict.get('Title');
       const flags = outlineDict.get('F') || 0;
       const color = outlineDict.getArray('C');
+      const count = outlineDict.get('Count');
       let rgbColor = blackColor;
 
       // We only need to parse the color when it's valid, and non-default.
@@ -159,7 +160,7 @@ class Catalog {
         newWindow: data.newWindow,
         title: stringToPDFString(title),
         color: rgbColor,
-        count: outlineDict.get('Count'),
+        count: Number.isInteger(count) ? count : undefined,
         bold: !!(flags & 2),
         italic: !!(flags & 1),
         items: [],

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