Add DecorationRangeBehavior.Word

[mjbvz]

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