Honor PSQL_EDITOR when opening the external editor

[ChrisJr404]

Description

Closes #1398. psql lets you point \e at a dedicated editor via PSQL_EDITOR so it doesn't have to be whatever EDITOR/VISUAL you use everywhere else, and pgcli didn't respect it.

pgcli hands editing off to pgspecial.open_external_editor, which calls click.edit() and lets click resolve the editor from VISUAL/EDITOR. That function already takes an editor= override, we just weren't passing anything. I added a small get_editor() helper that mirrors psql's lookup order (PSQL_EDITOR, then EDITOR, then VISUAL) and threaded its result through both editor call sites (\e/\ev/\ef and \ne). When none of those are set it returns None, which keeps click's existing platform-default behaviour, so nobody who wasn't setting PSQL_EDITOR sees a change.

Test-wise I added test_get_editor_precedence asserting PSQL_EDITOR wins over EDITOR/VISUAL, that it falls back to EDITOR then VISUAL, and returns None when the environment is empty. Ran python -m pytest tests/test_main.py -k "editor or named_query" (2 passed) and the full tests/test_main.py (60 passed, 10 db skips). ruff check and ruff format --diff are both clean.

Checklist

• I've added this contribution to the changelog.rst.
• I've added my name to the AUTHORS file (or it's already there).
• I installed pre-commit hooks (pip install pre-commit && pre-commit install).
• I verified that my changes work as expected.
• Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)

[dbaty]

Looks good, thank you! :)