feature: prepare the json schemas in repository

[byCedric]

Fixes #83

Linked issue

This implements the idea outlined in this comment.

Additional context

Instead of manually downloading and modifying the JSON Schemas in the plugin, we now do this inside this repository. The idea needs to be documented thoroughly, but here is the outline:

• Download and prepare the JSON Schemas in GitHub Actions
• Link the JSON Schema through contributionPoints.jsonValidation
• Remove all custom schema handling code from the plugin itself

TODO

• [x] Add tests for the new scripts/schema-xdl.js
• [x] Add EAS Metadata from latest published eas-cli version → See pull #108
• [x] Add documentation for this new system → See the schemas branch

[github-actions[bot]]

:tada: This PR is included in version 0.8.0 :tada:

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