Add NixOS module for fontconfig-ultimate

Details:
* The option `fonts.fontconfig.ultimate.enable` can be used to disable
  the fontconfig-ultimate configuration.
* The user-configurable options provided by fontconfig-ultimate are
  exposed in the NixOS module: `allowBitmaps` (default: true),
  `allowType1` (default: false), `useEmbeddedBitmaps` (default: false),
  `forceAutohint` (default: false), `renderMonoTTFAsBitmap` (default:
  false).
* Upstream provides three substitution modes for substituting TrueType
  fonts for Type 1 fonts (which do not render well). The default,
  "free", substitutes free fonts for Type 1 fonts. The option "ms"
  substitutions Microsoft fonts for Type 1 fonts. The option "combi"
  uses a combination of Microsoft and free fonts. Substitutions can also
  be disabled.
* All 21 of the Infinality rendering modes supported by fontconfig-ultimate
  or by the original Infinality distribution can be selected through
  `fonts.fontconfig.ultimate.rendering`. The default is the medium style
  provided by fontconfig-ultimate. Any of the modes may be customized,
  or Infinality rendering can be disabled entirely.