Select first item on file result header click

Created by: vdavid

Fixes https://github.com/sourcegraph/sourcegraph/issues/38293

I just set up a new "click" event handler on headers, and select the first item of the match if it's present.

Unrelated change: added a missing "role" to list items. (I'm unsure whether this matters inside the IDE, but it could help.)

Test plan

• half-minute Loom where I demo both the standalone and in-IDE versions

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