Stricter return type for makeValidator

[lostfictions]

This PR makes the return type for makeValidator stricter, helping to ensure that you're returning a value from the validator that at least looks right.

The caveat here is that JSON.parse returns any, so if that's your first step you'll have to use something like a user-defined typeguard (see isArrayOfStrings helper added to test file) to further refine the type if you want TS to catch anything. But this should still help if, say, you write makeValidator<number>(....) and try to return new Date(....) at the end of it.

[af]

👍 thanks!