Autocomplete improvements for tables

When completing tables (or table like things) the priority of completed items could be improved.
For one thing it makes no sense to offer columns as a completion after `FROM`, `JOIN` keywords.

I think a good list of options (sorted by priority) would be:

- CTE names
- tables/views that are directly visible, i.e. that belong to the `PATH`
- schemas
- functions

After the `UPDATE` keyword the list would not contain CTEs and functions.

No files, tests, or entry points are named. Start by tracing completion handling for FROM, JOIN, and UPDATE clauses, then verify that CTEs, visible tables or views, schemas, and functions appear in the requested priority order and that UPDATE excludes CTEs and functions.

Written by the indexing model from the issue text.