[component]: Search (header)

[gfellerph]

Create a standalone header search component. The current implementation (https://github.com/swisspost/design-system/tree/main/packages/internet-header/src/components/post-search) relies on a config file, the new search component should be independent of the config.

Design

https://www.figma.com/design/JIT5AdGYqv6bDRpfBPV8XR/Foundations-%26-Components-Next-Level?node-id=2908-36738&t=saHtog1amMMrG60S-4

Requirements

• Keep the functionality of the current header intact
• Suggestions can come from three places: track & trace, Coveo or places.post.ch
• The suggestions should be navigable by keyboard up and down arrows

Reference

### Tasks
- [ ] Create the `<post-search>` component
- [ ] Document this component in storybook
- [ ] Add e2e tests
- [ ] Add unit tests
- [ ] Add visual regression tests
- [ ] Design review