UI: Remove usage of private APIs

[aduth]

What problem does this address?

Using private APIs in bundled packages like @wordpress/ui (example) can cause issues for downstream projects that depend on them, due to the unavailability of private APIs, or the potential for multiple versions of the @wordpress/private-apis package due to bundling.

What is your proposed solution?

Avoid using private APIs in @wordpress/ui.

Options to consider:

• Recommended: Create a new @wordpress/theme/theme-provider exported entrypoint that's used internally or by external consumers using the npm package directly, i.e. not through the wp. window global.
  • Noting that this could still cause some duplication of the component with the one that would end up being shipped as the wp. window global (private) component.
• Alternative: Promote ThemeProvider to a public API.