fix: add checks and handle errors brew-bump.sh

feat(script): add steps-lib, is_env_var_set & test

feat(brew-bump): add check for VERSION

feat(brew-bump): check HOMEBREW_GITHUB_API_TOKEN

feat(steps-lib): add directory_exists helper fn

fix(brew-bump): check that git clone worked

feat(brew-bump): add check for remote upstream

fix: remove upstream command thing

feat(steps-lib): add file_exists helper function

feat(brew-bump): add check for git-askpass.sh

feat(steps-lib): add is_executable function & test

feat(brew-bump): add check for is_executable

refactor: use GIT_ASKPASS as variable