[w3c/editing] We should have more steps for Selection#extend() (#151)

We should describe following situation explicitly:
 - Call Selection#extend() with (null, int); 
  which one, throw, ignore or removeAllRanges()?
 - Call Selection#extend() when Seleciton#type is "None"
  which one throw, ignore, or collapse()?
 - Throw IndexSizeError for invalid offset



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