feat(react-instantsearch-ui): implement chat components design

[FabienMotte]

Summary

This PR implements the design for the chat components in InstantSearch.

The changes include:

• Added common chat UI components (Chat, ChatHeader, ChatMessage, ChatMessages, ChatPrompt, ChatToggleButton)
• Added new specialized components for error states and loading indicators (ChatMessageError, ChatMessageLoader)
• Restructured InstantSearch CSS architecture with modular SCSS files organized by component
• Responsive design / dark and light modes
• CSS custom variables for consistent theming and easy customization
• Added icon set for chat interface elements
• Improved TypeScript definitions and component APIs
• Added utility hooks like useStickToBottom (from https://github.com/stackblitz-labs/use-stick-to-bottom) for better chat UX
• Updated bundle size configurations to fit the new components/styles

Result

[Haroenv]

looks good!

[shaejaz]

lgtm once CI passes! Tested it with react and js versions locally, both working well (js has an issue with the loader not showing up but that might be js specific).

[FabienMotte]

lgtm once CI passes! Tested it with react and js versions locally, both working well (js has an issue with the loader not showing up but that might be js specific).

Thanks for the review! Could you please approve the PR? 🙏