Non-Boolean Additional Properties

[sinclairzx81]

This PR implements support for additionalProperties: TSchema on Value and TypeCompiler API's to allow for additional properties of a known type. This functionality aligns TypeBox validators with non-boolean additional properties constraints which are outlined at http://json-schema.org/understanding-json-schema/reference/object.html#additional-properties

You can use non-boolean schemas to put more complex constraints on the additional properties of an instance. For example, one can allow additional properties, but only if their values are each a string:

This PR also includes a new generic argument of TAdditionalProperties on TObject. This generic argument is used to infer a intersection of the normal TObject inference type with a Record<string, TAdditionalProperties> type. Intersection is only mapped when passing a non-boolean additionalProperties value. The TAdditionalProperties argument itself is static and reflectable.

Testing has been completed on TS 4.1.5 and should be fine; however the inference for TAdditionalProperties can be considered provisional as the output type produced results in a non-assignable type whose usage is somewhat limited. Future updates may default the inference to unknown.

const T = Type.Object({
  x: Type.Number(),
  y: Type.Number(),
}, {
  additionalProperties: Type.String()
})

function test(value: Static<typeof T>) {
  value.x = 1
  value.y = 1
  value.z = 'hello world' // ok
}