feat: add country selector screen (ACC-7226)

[OnurVar]

Summary

• New CountrySelectorScreen — full-screen picker with search, auto-focus, flag + name rows, pre-scrolled to current selection.
• Static ISO 3166-1 alpha-2 country list at `src/Components/internal/countries.ts` (+ `getCountryName` helper).
• `flagEmoji` helper using regional-indicator codepoints (no asset dep).
• Registers `CountrySelectorScreen` on `CheckoutRoute.countrySelector` in `CheckoutFlow`'s screen map.
• `CountrySelectorRow` and `PrimerBillingAddressForm` now render the flag + localized country name.

Design

• Figma: https://www.figma.com/design/fHmyhCfJZbwPoZRUc7Blqv/Checkout-Components-UI?node-id=1153-22046

Jira

https://primerapi.atlassian.net/browse/ACC-7226

Test plan

• `yarn typecheck` clean
• `yarn lint` exit 0
• iOS simulator: tap country row in billing form → picker opens with search focused → type "unit" → filters → pick country → pops back, row shows flag + name
• Re-open picker → selected country highlighted and scrolled into view
• Card + billing fields survive the push/pop round-trip
• Android emulator: same