Improve JSON spec
Improve JSON spec

For better verification of theme updates:

- add string within array
- add deeply nested object structure
- escaping within a string

colllin/one-dark-syntax,JeNeSuisPasDave/one-dark-syntax,nviel/nico-syntax,vith/one-dark-syntax,nviel/nico-syntax,JeNeSuisPasDave/one-dark-syntax,yuriihabrusiev/one-dark-syntax,JeNeSuisPasDave/one-dark-syntax,colllin/one-dark-syntax,grigio/atom-darkmate-syntax,JeNeSuisPasDave/one-dark-syntax,grigio/atom-darkmate-syntax,colllin/one-dark-syntax,nviel/nico-syntax,vith/one-dark-syntax,colllin/one-dark-syntax,yuriihabrusiev/one-dark-syntax,grigio/atom-darkmate-syntax,nviel/nico-syntax,yuriihabrusiev/one-dark-syntax,grigio/atom-darkmate-syntax,nviel/nico-syntax,yuriihabrusiev/one-dark-syntax,yuriihabrusiev/one-dark-syntax,nviel/nico-syntax,vith/one-dark-syntax,colllin/one-dark-syntax,vith/one-dark-syntax,grigio/atom-darkmate-syntax,JeNeSuisPasDave/one-dark-syntax,yuriihabrusiev/one-dark-syntax,grigio/atom-darkmate-syntax,vith/one-dark-syntax,vith/one-dark-syntax,colllin/one-dark-syntax,JeNeSuisPasDave/one-dark-syntax