Add basic support for non-embedded GillSansMT fonts (issue 18801)

Given the following excerpt from the [Wikipedia article](https://en.wikipedia.org/wiki/Gill_Sans), mapping this to Helvetica should hopefully be fine:

> It has been described as "the British Helvetica" because of its lasting popularity in British design.

src/core/standard_fonts.js

@@ -136,6 +136,10 @@ const getNonStdFontMap = getLookupTableFactory(function (t) {
   t["ComicSansMS-Bold"] = "Comic Sans MS-Bold";
   t["ComicSansMS-BoldItalic"] = "Comic Sans MS-BoldItalic";
   t["ComicSansMS-Italic"] = "Comic Sans MS-Italic";
+  t.GillSansMT = "Helvetica";
+  t["GillSansMT-Bold"] = "Helvetica-Bold";
+  t["GillSansMT-BoldItalic"] = "Helvetica-BoldOblique";
+  t["GillSansMT-Italic"] = "Helvetica-Oblique";
   t.Impact = "Helvetica";
   t["ItcSymbol-Bold"] = "Helvetica-Bold";
   t["ItcSymbol-BoldItalic"] = "Helvetica-BoldOblique";

test/pdfs/issue18801.pdf.link

@@ -0,0 +1 @@
+https://github.com/user-attachments/files/17150005/Test_FontIssue.2.pdf

test/test_manifest.json

@@ -3031,6 +3031,15 @@
     "lastPage": 1,
     "type": "eq"
   },
+  {
+    "id": "issue18801",
+    "file": "pdfs/issue18801.pdf",
+    "md5": "580258d9a282bfbec7b74da076bbccc9",
+    "link": true,
+    "rounds": 1,
+    "lastPage": 1,
+    "type": "eq"
+  },
   {
     "id": "issue11139",
     "file": "pdfs/issue11139.pdf",