Migrate surroundingPair to use a scope handler

[pokey]

We probably actually want three different versions of this scope handler (could prob just be flags on the scope type object itself)

• The usual one where the domain and content range are the full surrounding pair
• One where the domain is the full surrounding pair, but content range is "Inside", excluding whitespace
• One where the domain is "inside", including whitespace, and content range is "Inside", excluding whitespace

[AndreasArvidsson]

Implemented