[css-text] pre-wrap / pre-wrap-auto

Proposal:
   1. Remove the pre-wrap-auto value that we added to css3-text 5 months ago.
   2. Make pre-wrap UA-defined such that all of the following behaviors are allowed:
       a) wrapping after entire sequence of spaces
       b) wrapping after each space
       c) "visually collapsing" spaces that would otherwise need to be wrapped
       d) hanging trailing spaces (as in 'hanging-punctuation')
       e) anything else we think should be allowed
   3. Possibly recommend some specific behavior in level 4; I would suggest d).

~fantasai