Function optional

optional<T>(part: T): ShapeOptional<T>
Define a shape part that is optional. This only makes sense as a property in an object.
Type Parameters
- T
Parameters
- part: T
Returns ShapeOptional<T>
Example
```
import {optional, defineShape} from 'object-shape-tester';

const myShape = defineShape({
    a: optional(-1),
});

// `myShape.runtimeType` is `{a?: number}`
```
- Defined in src/define-shape/shape-specifiers.ts:435