Completions prefetching

[Saghen]

Feature Description

Prefetching can reduce latency to 0ms, removing the final bottleneck for blink.cmp's latency, the LSP itself!

With #697 merged, we now have a reliable way to pass the source of a completion request upstream, meaning completion.trigger.prefetch_on_insert = true seems to work reliably. Some ideas of when to prefetch:

• InsertEnter
• CursorHold? Only in insert mode?
• After pressing c in normal mode?
• After deleting characters onto a trigger character (a.b| -> a.|)
• Other ideas?

CC @stefanboca

[stefanboca]

Possibly on CmdlineEnter or CmdwinEnter for cmdline prefetching? #305 and #665 should also probably allow prefetching somehow, although I'm not sure if that's already handled by InsertEnter.