feat(completions): fill in alias for columns in join clauses

[juleswritescode]

Given a query

select 
  email, id
from
  auth.users u
  join auth.identities i on {}

if you pick user_id from the list, it will fill in i.user_id now.

[juleswritescode]

@psteinroe Should we also fill in aliases in select clauses?
It would be cool to do this only if we have duplicate columns, but that's odd to implement with the current design.
We could also simply always add the alias. WDYT?