[api-minor] Replace PDFDocumentProxy.getStats with a synchronous PDFDocumentProxy.stats getter

*Please note:* These changes will primarily benefit longer documents, somewhat at the expense of e.g. one-page documents.

The existing `PDFDocumentProxy.getStats` function, which in the default viewer is called for each rendered page, requires a round-trip to the worker-thread in order to obtain the current document stats. In the default viewer, we currently make one such API-call for *every rendered* page.
This patch proposes replacing that method with a *synchronous* `PDFDocumentProxy.stats` getter instead, combined with re-factoring the worker-thread code by adding a `DocStats`-class to track Stream/Font-types and *only send* them to the main-thread *the first time* that a type is encountered.

Note that in practice most PDF documents only use a fairly limited number of Stream/Font-types, which means that in longer documents most of the `PDFDocumentProxy.getStats`-calls will return the same data.[1]
This re-factoring will obviously benefit longer document the most[2], and could actually be seen as a regression for one-page documents, since in practice there'll usually be a couple of "DocStats" messages sent during the parsing of the first page. However, if the user zooms/rotates the document (which causes re-rendering), note that even a one-page document would start to benefit from these changes.

Another benefit of having the data available/cached in the API is that unless the document stats change during parsing, repeated `PDFDocumentProxy.stats`-calls will return *the same identical* object.
This is something that we can easily take advantage of in the default viewer, by now *only* reporting "documentStats" telemetry[3] when the data actually have changed rather than once per rendered page (again beneficial in longer documents).

---
[1] Furthermore, the maximium number of `StreamType`/`FontType` are `10` respectively `12`, which means that regardless of the complexity and page count in a PDF document there'll never be more than twenty-two "DocStats" messages sent; see 41ac3f0c07/src/shared/util.js (L206-L232)

[2] One example is the `pdf.pdf` document in the test-suite, where rendering all of its 1310 pages only result in a total of seven "DocStats" messages being sent from the worker-thread.

[3] Reporting telemetry, in Firefox, includes using `JSON.stringify` on the data and then sending an event to the `PdfStreamConverter.jsm`-code.
In that code the event is handled and `JSON.parse` is used to retrieve the data, and in the "documentStats"-case we'll then iterate through the data to avoid double-reporting telemetry; see https://searchfox.org/mozilla-central/rev/8f4c180b87e52f3345ef8a3432d6e54bd1eb18dc/toolkit/components/pdfjs/content/PdfStreamConverter.jsm#515-549

src/core/parser.js

@@ -741,13 +741,13 @@ class Parser {
       warn(`Empty "${name}" stream.`);
       return new NullStream();
     }
+    const xrefStats = this.xref.stats;
 
     try {
-      const xrefStreamStats = this.xref.stats.streamTypes;
       switch (name) {
         case "Fl":
         case "FlateDecode":
-          xrefStreamStats[StreamType.FLATE] = true;
+          xrefStats.addStreamType(StreamType.FLATE);
           if (params) {
             return new PredictorStream(
               new FlateStream(stream, maybeLength),
@@ -758,7 +758,7 @@ class Parser {
           return new FlateStream(stream, maybeLength);
         case "LZW":
         case "LZWDecode":
-          xrefStreamStats[StreamType.LZW] = true;
+          xrefStats.addStreamType(StreamType.LZW);
           let earlyChange = 1;
           if (params) {
             if (params.has("EarlyChange")) {
@@ -773,30 +773,30 @@ class Parser {
           return new LZWStream(stream, maybeLength, earlyChange);
         case "DCT":
         case "DCTDecode":
-          xrefStreamStats[StreamType.DCT] = true;
+          xrefStats.addStreamType(StreamType.DCT);
           return new JpegStream(stream, maybeLength, params);
         case "JPX":
         case "JPXDecode":
-          xrefStreamStats[StreamType.JPX] = true;
+          xrefStats.addStreamType(StreamType.JPX);
           return new JpxStream(stream, maybeLength, params);
         case "A85":
         case "ASCII85Decode":
-          xrefStreamStats[StreamType.A85] = true;
+          xrefStats.addStreamType(StreamType.A85);
           return new Ascii85Stream(stream, maybeLength);
         case "AHx":
         case "ASCIIHexDecode":
-          xrefStreamStats[StreamType.AHX] = true;
+          xrefStats.addStreamType(StreamType.AHX);
           return new AsciiHexStream(stream, maybeLength);
         case "CCF":
         case "CCITTFaxDecode":
-          xrefStreamStats[StreamType.CCF] = true;
+          xrefStats.addStreamType(StreamType.CCF);
           return new CCITTFaxStream(stream, maybeLength, params);
         case "RL":
         case "RunLengthDecode":
-          xrefStreamStats[StreamType.RLX] = true;
+          xrefStats.addStreamType(StreamType.RLX);
           return new RunLengthStream(stream, maybeLength);
         case "JBIG2Decode":
-          xrefStreamStats[StreamType.JBIG] = true;
+          xrefStats.addStreamType(StreamType.JBIG);
           return new Jbig2Stream(stream, maybeLength, params);
       }
       warn(`Filter "${name}" is not supported.`);