Relax font alias pattern to allow custom font families

Update the font alias regex pattern from:
    (mono|sans)-(bold-|italic-)?(stroke-)?[0-9]{1,3}
to:
    [a-z]+-(bold-|italic-)?(stroke-)?[0-9]{1,3}

Previously the pattern only allowed the `mono` and `sans` font families.
This change generalizes the pattern to accept any lowercase font family
name, making it compatible with the font alias convention described in:

https://gitea.wildfiregames.com/0ad/0ad/wiki/Adding_font_support

This allows new fonts to be registered through aliases without modifying
the regex each time a new family is introduced.

binaries/data/mods/mod/gui/gui.rng

@@ -89,7 +89,7 @@
   </define>
   <define name="font">
     <data type="string">
-      <param name="pattern">(mono|sans)-(bold-|italic-)?(stroke-)?[0-9]{1,3}</param>
+      <param name="pattern">[a-z]+-(bold-|italic-)?(stroke-)?[0-9]{1,3}</param>
     </data>
   </define>
   <define name="size">