Add "array" bloblang method

[gramian]

These changes add an array type coercion bloblang method similar to the bytes method: If the caller is already of type array the input is returned. If it is any other type the input is wrapped into a one-element array. Such a method helps to avoid constructs like this.with("name").values().flatten() to ensure an array returned.

• added passing tests
• format passing style
• added docs entry

Feedback welcome