tools like vscode have great support for includeing a `"$schema"`
property in your json for code completion and validation.

usually that `$schema` prop points to
`https://docs.renovatebot.com/renovate-schema.json` but this is
always the *latest* version of renovate.

in self-hosted environments not always the latest version is used,
so this patch adds the schema file to the bundled package for
easier reuse.

closes renovatebot#13835

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>