Create JSON Schema for Tiled json format

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**Is your feature request related to a problem? Please describe.**
- Get up-to-date C parser for tiled json files. Not sure which C parsers are up-to-date, whether they read all or some part of the file etc.

**Describe the solution you'd like**
- Create (and maintain) official JSON Schema and then use [quicktype](https://app.quicktype.io/?utm_source=awesome-jsonschema) to generate C code.
- Note that code can be generated for over 20 languages

**Describe alternatives you've considered**
- Three C libraries linked to in docs.

**Additional notes**
- Such approach is provided by ldtk [here](https://ldtk.io/docs/game-dev/loading/#2-the-quicktype-way) which I tested and works well
- Dedicated library might have more ergonomic API. The generated parser, for example, uses list to iterate over multiple elements (`list_get_head(ldtk->levels)`) instead of regular array access
- JSON Schema can contain description for various data which is later added to the code in form of comments.
- JSON Schema could be used to generate documentation, example: [here](https://github.com/adobe/jsonschema2md), and [here](https://github.com/coveooss/json-schema-for-humans)
- JSON Schema could be converted to XSD schema (if needed): [here](https://www.convertsimple.com/convert-json-schema-to-xsd/)
- There are tools to generate JSON Schema from json file, which could serve as a starting point: [here](https://www.jsonschema.net/)

Documentation of the json format has to exist in some form: html, markdown or JSON schema. Moving to adopting JSON schema might thus potentially be only one off cost.

Start by reviewing the existing Tiled JSON-format documentation and the three C libraries linked in the issue, then compare their coverage with the format. Done means an official, maintained JSON Schema exists and can serve as the basis for generating C code and format documentation.

Written by the indexing model from the issue text.