Extends Optional Property Check

[sinclairzx81]

This PR resolves an optional property check when comparing left and right property keys. The following now works.

type A = { a: number }
type B = { a?: number }
type C = A extends B ? true : false // true

// ...

const A = Type.Object({ a: Type.Number() })
const B = Type.Object({ a: Type.Optional(Type.Number()) })
const C = TypeExtends.Extends(A, B) // True

Inversion

type A = { a: number }
type B = { a?: number }
type C = B extends A ? true : false // false

// ...

const A = Type.Object({ a: Type.Number() })
const B = Type.Object({ a: Type.Optional(Type.Number()) })
const C = TypeExtends.Extends(B, A) // False