Check if an environment variable is set

[9999years]

In Bash, it's very common to check if an environment is set and non-empty with [[ -n "${VAR:-}" ]]. We should provide helpers for this, like so:

hasEnv :: Text -> IO Bool
hasEnv = not . missingEnv

missingEnv :: Text -> IO Bool
missingEnv name = null $ fromMaybe "" $ need name

[Gabriella439]

Let me know if https://github.com/Gabriella439/turtle/pull/454 looks okay to you