Autocompletion hangs adminer UI - fix or allow to disable

[sknaumov]

Adminer version: 5.4.1
Compiled: official docker image
Driver: PostgreSQL
Database version: PostgreSQL 18

Follow-up to #1142

I've used adminer to experiment with big query (~5k symbols) with multiple sub-queries.
I tried to delete ::numeric conversion in this query, but autocompletion hung for 65 seconds while deleting these 9 symbols, making editing queries in adminer UI impossible. I observed 100% CPU usage for single core during this period.

Please fix autocompletion engine, adjust it not to be applied for queries more than 1k characters or allow to disable it / tune query size parameter via env variable.

I had to fall back to v5.1.0, where there is no autocompletion. In this version there are no hangs.