fix: double escaped http-source selectors

[jone]

Problem

The current JSON contains double-escaped http-source selectors.

Invalid example ("id": "google-crawler"):

            "type": "http-json",
            "url": "https://developers.google.com/static/crawling/ipranges/common-crawlers.json",
            "selector": "$.prefixes[*][\\\"ipv6Prefix\\\",\\\"ipv4Prefix\\\"]"

The current file also contains a correct example ("id": "google-crawler-mobile"):

            "type": "http-json",
            "url": "https://developers.google.com/static/crawling/ipranges/common-crawlers.json",
            "selector": "$.prefixes[*][\"ipv4Prefix\",\"ipv6Prefix\"]"

This change fixes invalid selectors in the data.

Validation

I've added a specific validation for this problem. A better validation would be to actually parse the selector expression, but that would need an a dependency and a package.json. I did not want change the no-dependency principle of the project.

[davidmytton]

Thanks for the fix!