dotenv: add agree-once improvement to confirmation prompt (#8729)

* dotenv: add possibility to agree once for a given .env file

* refactor: fix code style

* Use :A modifier instead of readlink. Thanks Aloxaf

* Use grep and clean up allowed list check logic

* Simplify and reorder file; change default allowed list path

* Add new feature to README

* Make sure ZSH_CACHE_DIR is defined

* Resolve symlinks in $PWD before storing in allowed list

Co-authored-by: Aloxaf <bailong104@gmail.com>
Co-authored-by: Marc Cornellà <marc.cornella@live.com>