Add more non-standard ligatures in the glyphlist.js file (issue 15516)

Note that this PR only adds the "underscore"-variant of *actually existing* ligatures, however the referenced PDF document also uses a couple of non-standard ones (e.g. `ft`, `Th`, and `fh`) that we cannot easily support without larger changes (since they don't have official Unicode-entries).
Given that it's clearly the PDF document, and its fonts, that's the culprit here it's not entirely clear to me that we actually want to attempt a larger refactoring/rewriting of the `glyphlist.js` code, assuming it's even generally possible. Especially when this patch alone already improves our copy-paste behaviour when compared to both Adobe Reader and PDFium, and that this is only the *second* time this sort of bug has been reported.

test/pdfs/.gitignore

@@ -108,6 +108,7 @@
 !issue10542_reduced.pdf
 !issue10665_reduced.pdf
 !issue11016_reduced.pdf
+!issue15516_reduced.pdf
 !issue11045.pdf
 !bug1057544.pdf
 !issue11150_reduced.pdf

test/unit/api_spec.js

@@ -2417,6 +2417,23 @@ Caron Broadcasting, Inc., an Ohio corporation (“Lessee”).`)
       await loadingTask.destroy();
     });
 
+    // TODO: Change this to a `text` reference test instead.
+    //       Currently that doesn't work, since the `XMLSerializer` fails on
+    //       the ASCII "control characters" found in the text-content.
+    it("gets text content with non-standard ligatures (issue issue15516)", async function () {
+      const loadingTask = getDocument(
+        buildGetDocumentParams("issue15516_reduced.pdf")
+      );
+      const pdfDoc = await loadingTask.promise;
+      const pdfPage = await pdfDoc.getPage(1);
+      const { items } = await pdfPage.getTextContent();
+      const text = mergeText(items);
+
+      expect(text).toEqual("ffi fi ffl ff fl \x07 \x08 Ý");
+
+      await loadingTask.destroy();
+    });
+
     it("gets empty structure tree", async function () {
       const tree = await page.getStructTree();