introduced `thread-picker-modal.react` component. This diff just introduces the component, but it is not used anywhere at the moment
Next steps:
# Implement `ThreadPickerModal` component
# Clean up/get rid of old `ThreadPicker` component
---
Linear Issue: [[ https://linear.app/comm/issue/ENG-1848/implement-modal-design | ENG-1848 ]]
Design: [[ https://linear.app/comm/issue/ENG-1846/design-modal-ui-on-figma | ENG-1778 ]]