Add suggestion when setting the search_path

[fbdb]

Description

To be suggested autocompletion when setting the search_path, the keyword's suggestion "search_path TO" will be suggested on a "SET" token, only if the latter is the first token of the SQL statement (to differentiate it from the "UPDATE foo SET"). Then, available schemas are suggested (after the "TO" token).

Resolves #1511

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I'm open to suggestions if I did anything weirdly.

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Checklist

• [x] I've added this contribution to the changelog.rst.
• [x] I've added my name to the AUTHORS file (or it's already there).

• [x] I installed pre-commit hooks (pip install pre-commit && pre-commit install).
• [x] Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)