[api-minor] Support accessing both the original and modified PDF fingerprint

The PDF.js API has only ever supported accessing the original file ID, however the second one that (should) exist in *modified* documents have thus far been completely inaccessible through the API.
That seems like a simple oversight, caused e.g. by the viewer not needing it, since it really shouldn't hurt to provide API-users with the ability to check if a PDF document has been modified since its creation.[1]

Please refer to https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf#G13.2261661 for additional information.

For an example of how to update existing code to use the new API, please see the changes in the `web/app.js` file included in this patch.

*Please note:* While I'm not sure if we'll ever be able to remove the old `PDFDocumentProxy.fingerprint` getter, given that it's existed since "forever", that probably isn't a big deal given that it's now limited to only `GENERIC`-builds.

---
[1] Although this obviously depends on the PDF software following the specification, by updating the second file ID as intended.

src/core/document.js

@@ -1153,30 +1153,44 @@ class PDFDocument {
     return shadow(this, "documentInfo", docInfo);
   }
 
-  get fingerprint() {
-    let hash;
+  get fingerprints() {
+    function validate(data) {
+      return (
+        typeof data === "string" &&
+        data.length > 0 &&
+        data !== EMPTY_FINGERPRINT
+      );
+    }
+
+    function hexString(hash) {
+      const buf = [];
+      for (let i = 0, ii = hash.length; i < ii; i++) {
+        const hex = hash[i].toString(16);
+        buf.push(hex.padStart(2, "0"));
+      }
+      return buf.join("");
+    }
+
     const idArray = this.xref.trailer.get("ID");
-    if (
-      Array.isArray(idArray) &&
-      idArray[0] &&
-      isString(idArray[0]) &&
-      idArray[0] !== EMPTY_FINGERPRINT
-    ) {
-      hash = stringToBytes(idArray[0]);
+    let hashOriginal, hashModified;
+    if (Array.isArray(idArray) && validate(idArray[0])) {
+      hashOriginal = stringToBytes(idArray[0]);
+
+      if (idArray[1] !== idArray[0] && validate(idArray[1])) {
+        hashModified = stringToBytes(idArray[1]);
+      }
     } else {
-      hash = calculateMD5(
+      hashOriginal = calculateMD5(
         this.stream.getByteRange(0, FINGERPRINT_FIRST_BYTES),
         0,
         FINGERPRINT_FIRST_BYTES
       );
     }
 
-    const fingerprintBuf = [];
-    for (let i = 0, ii = hash.length; i < ii; i++) {
-      const hex = hash[i].toString(16);
-      fingerprintBuf.push(hex.padStart(2, "0"));
-    }
-    return shadow(this, "fingerprint", fingerprintBuf.join(""));
+    return shadow(this, "fingerprints", [
+      hexString(hashOriginal),
+      hashModified ? hexString(hashModified) : null,
+    ]);
   }
 
   _getLinearizationPage(pageIndex) {

src/core/worker.js

@@ -199,9 +199,9 @@ class WorkerMessageHandler {
           .then(() => finishWorkerTask(task));
       }
 
-      const [numPages, fingerprint] = await Promise.all([
+      const [numPages, fingerprints] = await Promise.all([
         pdfManager.ensureDoc("numPages"),
-        pdfManager.ensureDoc("fingerprint"),
+        pdfManager.ensureDoc("fingerprints"),
       ]);
 
       // Get htmlForXfa after numPages to avoid to create HTML twice.
@@ -209,7 +209,7 @@ class WorkerMessageHandler {
         ? await pdfManager.ensureDoc("htmlForXfa")
         : null;
 
-      return { numPages, fingerprint, htmlForXfa };
+      return { numPages, fingerprints, htmlForXfa };
     }
 
     function getPdfManager(data, evaluatorOptions, enableXfa) {

src/display/api.js

@@ -722,6 +722,18 @@ class PDFDocumentProxy {
   constructor(pdfInfo, transport) {
     this._pdfInfo = pdfInfo;
     this._transport = transport;
+
+    if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
+      Object.defineProperty(this, "fingerprint", {
+        get() {
+          deprecated(
+            "`PDFDocumentProxy.fingerprint`, " +
+              "please use `PDFDocumentProxy.fingerprints` instead."
+          );
+          return this.fingerprints[0];
+        },
+      });
+    }
   }
 
   /**
@@ -739,10 +751,13 @@ class PDFDocumentProxy {
   }
 
   /**
-   * @type {string} A (not guaranteed to be) unique ID to identify a PDF.
+   * @type {Array<string, string|null>} A (not guaranteed to be) unique ID to
+   *   identify the PDF document.
+   *   NOTE: The first element will always be defined for all PDF documents,
+   *   whereas the second element is only defined for *modified* PDF documents.
    */
-  get fingerprint() {
-    return this._pdfInfo.fingerprint;
+  get fingerprints() {
+    return this._pdfInfo.fingerprints;
   }
 
   /**