Implement `font-feature-settings` and `font-variant` properties

[nicoburns]

Both of these CSS properties are:

• Supported by Stylo, but that support is currently disabled in servo builds of Stylo
• Support by Parley

The format that Stylo computes and the format that Parley accepts are pretty much identical (but may need some trivial conversion), so we just need to enable the properties in Stylo and then plug them into Parley..