nixos/fontconfig: Allow setting default emoji font

In fontconfig’s 60-generic.conf, order of preference is estabilished for emoji
font family. Because fontconfig parses the config files in lexicographic order,
appending each <prefer> from <alias> element to the family’s prefer list
(to be prepended before the family) [1], our font family defaults stored
in 52-nixos-default-fonts.conf will take precedence. That is, of course, unless
the default „weak“ binding [2] is used. Emoji family binds strongly [3],
so we need to set binding to “same” for our <alias>es to be considered before
the ones from 60-generic.conf.

By default, we will set the option to all emoji fonts supported by fontconfig,
so that emoji works for user if they have at least one emoji font installed.
If they have multiple emoji fonts installed, we will use the fontconfig’s
order of preference [4].

[1]: https://github.com/bohoomil/fontconfig-ultimate/issues/51#issuecomment-64678322
[2]: https://www.freedesktop.org/software/fontconfig/fontconfig-user.html#AEN25
[3]: https://gitlab.freedesktop.org/fontconfig/fontconfig/commit/cc8442dec85e9d416436d19eeae1783f2d3008f0
[4]: https://gitlab.freedesktop.org/fontconfig/fontconfig/commit/c41c9220181b203d1cf1f6435f6e3735cb7c84ac