Treat matching on the boundaries as an assertion

What the headline says ;) 

Also, this PR changes the `sub` property to `name` on the `type: assertion`. This is to make it more consistent with the `type:escaped` entry, which also has a `name` property.

I am not too much satisfied with the name `not-boundary`. Any better ideas are welcome :)
Ah, perfect! Sorry, should’ve checked this first. Forget what I said :)