[cssom] Environment encoding for xml-stylesheet PIs

Hi,

I just added the concept of "environment encoding" to CSS Syntax, and 
defined it there for xml-stylesheet processing instructions:

http://dev.w3.org/csswg/css-syntax/#environment-encoding
http://dev.w3.org/csswg/css-syntax/#environment-encoding-xml

Given that http://www.w3.org/TR/xml-stylesheet/ is carved in REC stone 
and CSSOM already has a section on xml-stylesheet, this should probably 
be moved there.

-- 
Simon Sapin