Fix return type annotations for the random() method

[DavidTimms]

The random() method on a type generates a random value conforming to the type of the schema. The TypeScript type definitions for this method were incorrect - they claimed that this method returned the type object itself. For instance, calling random on a BooleanType returns a boolean, not a BooleanType. I assume this has gone undetected for years, because this method is rarely used.

Because the types for schemas do not carry complex type information in generics, many of these return types are rather imprecise. I have had to resort to unknown quite often, because more exact types are not available. I have chose unknown over any, to maintain type safety.

[mtth]

Thank you for the fix. Just a quick comment.

[DavidTimms]

@mtth Thanks for merging this.

Do you have any plans for a release in the near future? I noticed there hasn't been a release in the a couple of years, so there are a few unreleased fixes built up now.