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 :)
Good point to raise this :)

I think the line 63 in the input file already contains this:

```
  "[\\B]",
```

The output is:

```
  {
    "type": "error",
    "name": "SyntaxError",
    "message": "\\B not possible inside of CharacterClass",
    "input": "[\\B]"
  },
```

Can you confirm this is what you had in mind?