Continue filling in the list of FontFeatures

This adds support and documentation for:

 * FontFeature.alternative(n)
 * FontFeature.alternativeFractions()
 * FontFeature.contextualAlternates()
 * FontFeature.caseSensitiveForms()
 * FontFeature.characterVariant(v)
 * FontFeature.denominator()
 * FontFeature.fractions()
 * FontFeature.historicalForms()
 * FontFeature.historicalLigatures()
 * FontFeature.liningFigures()
 * FontFeature.localeAware(enable: false)
 * FontFeature.notationalForms(v)
 * FontFeature.numerators()
 * FontFeature.oldstyleFigures()
 * FontFeature.ordinalForms()
 * FontFeature.proportionalFigures()
 * FontFeature.randomize() (minor additions to documentation)
 * FontFeature.stylisticAlternates()
 * FontFeature.scientificInferiors()
 * FontFeature.stylisticSet(n) (documentation)
 * FontFeature.subscripts()
 * FontFeature.superscripts()
 * FontFeature.swash(v)
 * FontFeature.tabularFigures() (documentation)
 * FontFeature.slashedZero()

There's hundreds more we could add but this is what I have so far.

See flutter/assets-for-api-docs#131 for the diagrams.

Author: Hixie