feat(trigger): prefetch on InsertEnter

[stefanboca]

Mostly a proof of concept, and probably missing some edge cases. Only prefetches completion responses from sources. Fuzzy still occurs on the first regular trigger, but there is still noticeably less latency.

[stefanboca]

Alternatively, the check for trigger.prefetch could be moved from the trigger show_emitter to the list show_emitter to prefetch results as well as fuzzy ahead of time, although I believe this would only help in the case where a completion is manually triggered directly following an InsertEnter.