feat: Add public useI18n hook

[radsacha]

Summary

Add a public namespace-aware useI18n(namespace, component) hook from @cloudscape-design/components/i18n.

This lets component libraries built on top of Cloudscape resolve their own translated strings through the existing I18nProvider message context, without exposing InternalI18nContext or changing Cloudscape's internal useInternalI18n hook.

Changes

• Add useI18n(namespace, component) as a thin public wrapper around the existing formatter.
• Export useI18n and I18nFormatFunction from @cloudscape-design/components/i18n.
• Add unit coverage for custom namespaces, provided-value precedence, no-provider fallback, locale widening, custom handlers, multiple namespaces, nested providers, and shared Cloudscape/third-party provider usage.

Non-goals

• No I18nProvider behavior changes.
• No message format or bundle shape changes.
• No changes to useInternalI18n or Cloudscape component typing.

Testing

• npm run quick-build
• node_modules/.bin/eslint src/i18n/context.ts src/i18n/index.ts src/i18n/__tests__/use-i18n.test.tsx
• TZ=UTC node_modules/.bin/jest -c jest.unit.config.js src/i18n/__tests__/use-i18n.test.tsx src/i18n/__tests__/i18n.test.tsx
• TZ=UTC node_modules/.bin/jest -c jest.unit.config.js src/__tests__/functional-tests/public-exports.test.ts
• git diff --check