Add detailed rules for the ssty OpenType feature

Currently, the ssty feature is mentioned in the following paragraph:
"Some characters like primes already have script size by default and hence would be too small when used in a script position. Hence user agents must support glyph selections via the OpenType font feature ssty (Script Style) in order to display such “prescripted” characters with the appropriate size." (chapter 2)
"If mtext [or token element with similar box model] is used at a non-zero scriptlevel then user agents must enable the ssty OpenType feature on text nodes unless it contradicts what the page author has specified with the font-feature-settings CSS property." (chapter 3)
See also:
https://bugzilla.mozilla.org/show_bug.cgi?id=442637
http://www.microsoft.com/typography/otspec/features_pt.htm#ssty
http://www.microsoft.com/typography/otspec/math.htm

Original report: https://gitlab.com/mathml/MathMLinHTML5/issues/2

Start by reviewing the existing ssty references in chapters 2 and 3, then compare them with the linked OpenType feature specifications and Mozilla report. Define the detailed rules for enabling ssty, including the interaction with font-feature-settings, and update the MathML Core draft so the behavior is unambiguous.