Make sure we correctly support additionalProperties set to True

[yvan-sraka]

This PR is a quick and dirty fix for #150. It makes the example highlighted in the issue work as expected but breaks other tests doing so...

import json
import re
from outlines_core.json_schema import build_regex_from_schema
schema = {
    "type": "object",
    "properties": {
        "a": {
            "type": "string",
        }
    },
    "additionalProperties": True
}
regex_str = build_regex_from_schema(json.dumps(schema))
data = json.dumps({
    "a": "aaa",
    "id": "1"
})
print(re.match(regex_str, data))

The overall design of JSON Schema parsing here is fragile and difficult to extend without a full refactoring... So, I've added a few comments to highlight the problem and suggest what could be a better implementation!