Fail on escape sequences in metadata.json

[bastelfreak]

This is an enhancement of #120

We now use a regex to identify invalid escape sequences. JSON.parse()
has no built-in way to detect this. Even in strict mode it ignores it:

irb(main):002> require 'json'
=> true
irb(main):003> JSON.parser = JSON::Ext::Parser
=> JSON::Ext::Parser
irb(main):004> JSON.parse('{"summary": "A description with an invalid \( escape sequence"}')
=> {"summary"=>"A description with an invalid ( escape sequence"}
irb(main):005> JSON.parse("{\"summary\": \"A description with an invalid \( escape sequence\"}")
=> {"summary"=>"A description with an invalid ( escape sequence"}
irb(main):006>

[ekohl]

I wondered if this really was invalid, but it looks like Ruby's JSON parser just silently ignores it while Python raises a hard JSONDecodeError so 👍 for enforcing it as an error. https://jsonlint.com/ also loudly complains about "\(".

[bastelfreak]

I wondered if this really was invalid
me too, but jq also complains:

bastelfreak@bastelfreak-nb ~/code/metadata-json-lint $ jq . tests/invalid_escape_char/metadata.json
jq: parse error: Invalid escape at line 5, column 63
bastelfreak@bastelfreak-nb ~/code/metadata-json-lint $