Write linter to check data files

[EmilyGraceSeville7cf]

• check extension
• check JSOM schema
• check urls
• check repeated entities

[EmilyGraceSeville7cf]

There is no need in such validator now, as everything is checked inside Go templates and via JSON schema.

[EmilyGraceSeville7cf]

I guess I can move such checks from Hugo templates to a separate tool. It will speed up site built, but at the same time allow CI to check whether everything is fine with data files. And as a bonus intellisence will be available, while in Hugo templates just syntax highlighting and snippets implemented yet.