Add JSON schema to validate structure of mods.json

[GeckoEidechse]

A JSON schema is a JSON file that describes the structure of a JSON file.

Defining a JSON schema would allow us to automatically verify the structure of the mod.json and automatically throw an error in CI if someone PRs an incorrect JSON file ^^

See also https://json-schema.org/overview/what-is-jsonschema

[Alystrasz]

On it

[GeckoEidechse]

Closed by #11