bug: defaults are not working correctly

The `defaults` option is not working correctly.

In the following example, I have two approaches, both of which are faulty.

### should fill in defaults with require

In this case, I am expecting the default to be set to 0, and then mark it as required, thereby eliminating the `undefined` from the resulting inferred type. Without the `require` at the end, `num` will have `number | undefined` type.

### should fill in defaults without require

Here's an example without require at the end. When doing the addition in the last step, a TypeScript error is produced, because the type is `number | undefined`:

> `Object is possibly 'undefined'. ts(2532)`

---

```ts
import { compile, v } from 'suretype'

describe('suretype', () => {
it('should fill in defaults with require', () => {
expect.assertions(1)

const schema = v.object({
num: v.number().default(0).required(),
})

const validator = compile(schema, { ensure: true })

const result = validator({})

expect(result.num).toStrictEqual(0)
})

it('should fill in defaults without require', () => {
expect.assertions(1)

const schema = v.object({
num: v.number().default(0),
})

const validator = compile(schema, { ensure: true })

const result = validator({})

expect(result.num + 1).toStrictEqual(1)
})
})
```

---

```
FAIL test/valid.test.ts
suretype
✕ should fill in defaults with require (39 ms)
✕ should fill in defaults without require (7 ms)

● suretype › should fill in defaults with require

Validation failed

11 | const validator = compile(schema, { ensure: true })
12 |
> 13 | const result = validator({})
| ^
14 |
15 | expect(result.num).toStrictEqual(0)
16 | })

at validate (node_modules/suretype/dist/json-schema.js:45:19)
at Object. (test/valid.test.ts:13:20)

● suretype › should fill in defaults with require

expect.assertions(1)

Expected one assertion to be called but received zero assertion calls.

3 | describe('suretype', () => {
4 | it('should fill in defaults with require', () => {
> 5 | expect.assertions(1)
| ^
6 |
7 | const schema = v.object({
8 | num: v.number().default(0).required(),

at Object. (test/valid.test.ts:5:12)

● suretype › should fill in defaults without require

expect(received).toStrictEqual(expected) // deep equality

Expected: 1
Received: NaN

27 | const result = validator({})
28 |
> 29 | expect(result.num + 1).toStrictEqual(1)
| ^
30 | })
31 | })
32 |

at Object. (test/valid.test.ts:29:28)
```