search: rename search type LiteralDefault -> Literal
Created by: rvantonder
As in title. This undos https://github.com/sourcegraph/sourcegraph/pull/35243 so we can implement RFC 675 with fewer potential disruptions.
More context: https://sourcegraph.slack.com/archives/C020S8AT3LN/p1655833010510029
Test plan
Semantics-preserving