zoekt: implement mode which has same behaviour as attribution search

[keegancsmith]

This is to help reproduce slow attribution searches we have on sourcegraph.com.

$ echo -n "const data: Event = { ...JSON.parse(message.data), type: message.event }" | go run ./cmd/zoekt -v -exact_stdin -index_dir ~/zoekt-dotcom-subset-2023-06-29 -cpu_profile profile.cpu
2023/07/13 09:54:26 loading 854 shard(s): github.com%2F214140846%2FTOGO_School_Miniprograme_v16.00000.zoekt, github.com%2F2fd%2Fgraphdoc_v16.00000.zoekt, github.com%2F5t3ph%2Fa11y-color-tokens_v16.00000.zoekt, github.com%2F97-things%2F97-things-every-programmer-should-know_v16.00000.zoekt, github.com%2FADmad%2Fcakephp-jwt-auth_v16.00000.zoekt... 849 more
2023/07/13 09:54:26 query: case_content_substr:"const data: Event = { ...JSON.parse(message.data), type: message.event }"
2023/07/13 09:54:26 Displaying matches...
client/cody-shared/src/sourcegraph-api/completions/browserClient.ts:48:                const data: Event = { ...JSON.parse(message.data), type: message.event }
2023/07/13 09:54:27 stats: zoekt.Stats{ContentBytesLoaded:81980, IndexBytesLoaded:441, Crashes:0, Duration:1481452, FileCount:1, ShardFilesConsidered:0, FilesConsidered:5, FilesLoaded:5, FilesSkipped:0, ShardsScanned:12, ShardsSkipped:0, ShardsSkippedFilter:841, MatchCount:1, NgramMatches:5, NgramLookups:11954, Wait:2860, RegexpsConsidered:0, FlushReason:0x0}