Currently, `tea` only supports the $EDITOR env var to open the user's
preferred editor (used for reviewing pull requests).
Standard *nix practice is, however, to check for $VISUAL first and only
use $EDITOR as fallback.
This is also done by Git itself, see man git-var(1).
(Actually, the order is $GIT_EDITOR > core.editor > $VISUAL > $EDITOR > vi)