Check if an environment variable is set

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:


```haskell
hasEnv :: Text -> IO Bool
hasEnv = not . missingEnv

missingEnv :: Text -> IO Bool
missingEnv name = null $ fromMaybe "" $ need name
```