search query input: Rethink search query suggestions UI and behavior

[fkling]

This is a placeholder issue, more concrete information will follow. CodeMirror's suggestions (autocompletion) looks and behaves differently than Monaco's. Rather than replicating Monaco's behavior (which might not be possible), we should use this opportunity to rethink how we want suggestions to work. A couple of questions that we might want to answer:

• What problem(s) are we trying to solve by showing suggestions?
• What are the different types of suggestions?
• What information do we want to display for every suggestion?
• How to coordinate between client-side (static) and server suggestions?
• How do we want to rank/order suggestions?
• What should happen when a suggestion is selected? Is there more we can do to help the user than just completing the word (e.g. add additional filters)?

[sourcegraph-bot-2]

Heads up @benvenker @lguychard - the "team/search-product" label was applied to this issue.

[fkling]

I'm removing the codemirror label because this is not specific to codemirror. @benvenker feel free to close this if you have another issue to keep track of future suggestions work.