validate .typesafe-i18n.json config file

[ivanhofer]

Currently the contents of the .typesafe-i18n.json file gets not validated.

• It would be a improvement if the generator throws an error if something is wrong e.g. if the outputFormat was set to javascript instead of JavaScript.
• If present the $schema attribute should be replaced with the actual version of the generator if an older version number is present in it's url

[ivanhofer]

implemented in version 2.52.0