feat: add `Readonly` utility

[ForbesLindesay]

This supports Object, Array, Tuple and Record. I have not removed the asReadonly utilities from Object or Array, but this Readonly helper should probably be preferred going forwards as it will lead to a lower bundle size because it can be shared among all types.

[closes #50]

[rollingversions[bot]]

Change Log for funtypes (4.0.0 → 4.1.0)

New Features

• Added Readonly helper type

  The Readonly helper type can be passed an Object, Array, Tuple or Record funtype and will convert it into the read only equivalent. The Object and Array types still have the asReadonly() method, but you should use Readonly(...) for new code as the asReadonly() method may eventually be removed to reduce the bundle size

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