[component]: Language switch

[gfellerph]

Create a standalone language switch component. The current implementation (https://github.com/swisspost/design-system/tree/main/packages/internet-header/src/components/post-language-switch) relies on a config file, the new component should be usable without that configuration.

Design

https://www.figma.com/design/JIT5AdGYqv6bDRpfBPV8XR/Foundations-%26-Components-Next-Level?node-id=2908-30439&t=saHtog1amMMrG60S-4

Requirements

• Keep the current functionality intact
• Arrow keys should be used to switch between languages
• Pay attention to the accessibility features implemented (screen readers should not read "EN" but "English" in the correct language)
• There are two ways the component can be displayed, as a dropdown and as a horizontal list
• This component does not need to be tokenised for the time being

Reference

### Tasks
- [ ] https://github.com/swisspost/design-system/issues/1242
- [ ] Create a `<post-language-switch>` component
- [ ] Add documentation
- [ ] e2e tests
- [ ] Visual regression tests
- [ ] Design review