New reference panel: duplicate more code-intel-extensions logic

Created by: mrnugget

This implements a bunch of things from https://github.com/sourcegraph/sourcegraph/issues/30970:

• First search in current repository, at current commit
• Then fallback to repositories that are not current repository, if not on Sourcegraph.com
• Race between indexed/unindexed search
  • If basicCodeIntel.indexOnly is set, do not search unindexed
  • Use basicCodeIntel.unindexedSearchTimeout when searching unindexed search

by using 95% copy&pasted code from code-intel-extensions, which, in this case, is a good thing! Because it means we don't have to duplicate a lot of hairy logic.

Test plan

• Tested locally. With network throttling on/off to check for "racing" between indexed/unindexed. Manually inspected requests.

App preview:

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