Autocompletion: implement smart autocompletion

[qdzo]

We need more smart-autocompletion, depending on context.

• complete keys for schema, bassed on current type
• complete keys for schema, based on current type confirmation (zen/vector confirms zen/coll)
• complete symbols from current ns, imported ns, zen.edn
• complete tags when cursor in :zen/tags #{}
• complete schemas when cursor in :confirms #{}