Allow record to produce readonly properties when provided readonly

[dubzzz]

🚀 Feature Request

At the moment fc.record only produce editable (aka writable) instances. In reality it could mirror what the user passed to it. If the received model is made of readonly fields, then they should probably stay readonly in the resulting instances (typings and js-wise). Same for sealed.

Before starting this feature, we should probably ask ourselves whether or not readonly should be yet another flag or something as described above.