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
