feat: generate theme.json meta-file on theme creation

[Velka-DEV]

This PR is about creating a theme.json file that provide informations about the create theme.

Here is the actual format proposed:

{
    "name": "Tailwind",
    "author": "Velka",
    "description": "",
    "version": "1.0.0",
    "thumbnail": null
}

This is specially used to create theme selector in applications.