fontconfig: Set up fallback fonts

In case a glyph is absent in the default font, st uses font-config and
tries to find them in font cache first. This also seems to work for all
VTE terminals. However, alacritty relies on system configuration and does
not do the above. To fix this, we need specify a fallback font manually.
This also seems good practice in general otherwise.

For reference, see the below links.
https://st.suckless.org/patches/font2/
https://github.com/alacritty/alacritty/issues/3690
https://wiki.archlinux.org/index.php/Fonts#Fallback_font_order
https://eev.ee/blog/2015/05/20/i-stared-into-the-fontconfig-and-the-fontconfig-stared-back-at-me/

Also our use of galaxyline plugin for neovim requires nvim-web-devicons
which in turn requires a patched font like Nerd font.

fontconfig/.config/fontconfig/fonts.conf

@@ -0,0 +1,18 @@
+<?xml version="1.0"?>
+<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
+<fontconfig>
+   <alias>
+      <family>JetBrains Mono</family>
+      <default>
+         <family>monospace</family>
+      </default>
+   </alias>
+   <alias>
+      <family>monospace</family>
+      <prefer>
+        <family>JetBrains Mono</family>
+        <family>TerminessTTF Nerd Font</family>
+      </prefer>
+   </alias>
+</fontconfig>
+