smartSearch doesn't recognize index field

Created by: rvantonder

Squiggles for index :-)

index isn't documented on the search syntax page, so I'll improve this (we should have a source of truth for all fields somewhere). The current definitive source is here, though some are experimental:

https://sourcegraph.com/github.com/sourcegraph/sourcegraph@40de355cfb7144a1888335ac35315ccf49fc0ce6/-/blob/internal/search/query/searchquery.go#L15-42

I think the main ones to still add are index and max. Aside, I think index should be promoted to a 'supported' field and not experimental. I would not, e.g., add codemod or rule since these are intentionally undocumented and not supported (and codemod will probably disappear soon). Will go through these more carefully in the coming days.