Web: 3.12 tracking issue

Created by: lguychard

OKR: 2.b.iv - Make the power of search accessible to more users

Planned work for 3.12:

• Finish RFC 14: Showing users suggestions of filters & filter values as they type into the search bar, and highlighting valid filters in search queries.

  • Implement Monaco-editor backed search query input, providing syntax highlighting, hover tooltips and diagnostics for search queries: https://github.com/sourcegraph/sourcegraph/pull/7669

• RFC 15: split the search UX into two modes, omni and interactive.

  • Add filter negation to text-based filter inputs (repo, file)
  • Add buttons for remaining filters
  • Add unit tests