Add .glif to XML file-types (#9130)

`.glif` files are standard files in the type design industry. From the
Unified Font Object specification website:

The Glyph Interchange Format (GLIF) is a simple and clear XML
representation of a single glyph. GLIF files typically have a .glif
extension.
https://unifiedfontobject.org/versions/ufo3/glyphs/glif/

languages.toml

@@ -2320,7 +2320,8 @@ file-types = [
   "xpdl",
   "xul",
   "xoml",
-  "musicxml"
+  "musicxml",
+  "glif"
 ]
 indent = { tab-width = 2, unit = "  " }