Add DecorationRangeBehavior.Word

From #50779 **Problem** Using decorators, a common pattern is that you want an decoration that applies to a given whole word. None of the current `DecorationRangeBehavior` values handle this well. `Open` will always expand the decorator even if you type non-word characters, while `Close` never expands the decorator **Proposal** Add a `DecorationRangeBehavior.Word` setting or similar that would expand the decoration if the edit extends the word at the edges of the decoration. Typing a word character would expand the decorator while typing space would not