Add POS Resolution API types

[henryStelle]

Summary

• add private POS ResolutionApi and ResolutionApiContent types
• expose the intercepted workflow as shopify.resolution.event
• type event as a read-only signal whose value is the union of ShopifyInterceptMap events
• add a minor changeset for @shopify/ui-extensions

Part of https://github.com/shop/issues-retail/issues/33117

This intentionally does not register or modify any extension target.

Dependency

• Shopify/extensibility#1631 depends on this PR.

Validation

• yarn build
• yarn type-check
• yarn lint
• yarn test -- packages/ui-extensions/ is currently blocked by the repository's missing react-test-renderer dependency (2 suites pass; 36 unrelated checkout suites fail during loading).