refs https://github.com/TryGhost/Product/issues/4152
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### <samp>🤖[[deprecated]](https://githubnext.com/copilot-for-prs-sunset)
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This pull request adds a new admin-x app called `admin-x-demo`, which
demonstrates how to use the shared packages `admin-x-framework` and
`admin-x-design-system` to create a simple app that renders a button and
a modal. It also improves the development workflow, the vite
integration, the dependency management, and the type checking for the
admin-x apps and packages. It modifies some files in the
`admin-x-framework` and `admin-x-design-system` packages to make the
modals prop optional, to introduce a new type for the props from the
Ember app, to fix the z-index of the modal backdrop, and to use
consistent file extensions and module syntax.
