Add support for "next of type" / "previous of type" / "every of type"

[pokey]

This stage will inspect the incoming target to get its scope type, then apply "every" / "next" / whatever using that scope type. Not sure of spoken form, but will be useful for #991.

• Add proper scopeType attribute to targets, instead of scopeTypeType
• Add stage which is thin wrapper around existing RelativeScopeStage / OrdinalScopeStage that gets scope type from target. Could even just integrate it into these stages and make scopeType optional on descriptor to imply it should come from target