[META] Move Elasticsearch API spec to Elasticsearch repo

[technige]

This repository contains two distinct parts:

1. The Elasticsearch API spec source and output
2. The tooling to manage API specifications in general (validation and compilation)

The task here is to migrate the first part into the Elasticsearch repository so that it can be owned close to the server source code.

The end goal should include packaged tooling that can be used by the Elasticsearch developers (e.g. in a Docker image). No specific knowledge of TypeScript and the JS stack should be needed.

TODO: detailed list of work items