Add CI workflow for Schema Store PR submission

[nokome]

Add GitHub Actions workflow to automatically submit PRs to JSON Schema Store.

Context

JSON Schema Store (https://www.schemastore.org/) provides schema autocompletion in editors. Submitting OXA schema enables IDE support for OXA documents.

Reference

Similar to: https://github.com/stencila/stencila/blob/main/.github/workflows/release-schemastore.yml

Prerequisites

• JSON Schema generation must be complete Add JSON Schema generation from YAML #7
• Documentation must be published (Schema Store requires a docs URL) (forthcoming issue)

Tasks

• Create .github/workflows/schemastore.yml
• Trigger on releases or manual dispatch
• Fork/clone schemastore/schemastore
• Update OXA schema entry
• Submit PR automatically