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 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";