Knockout not exported from Cesium.d.ts

### What happened?

`knockout` is exported from `Cesium.js`, but there is no matching export within `Cesium.d.ts`. Therefore there is no obvious type-safe way to access knockout functionality, such as subscribing to observables within view models.

The work around is to disable typescript checking of the relevant line:

```
// @ts-ignore
import { knockout as ko } from 'cesium';
```

TS type definitions exist for both knockout-es5 and knockout:

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/knockout.es5/index.d.ts
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/knockout/index.d.ts

### Environment

CesiumJS Version: 1.123.1

Compare the export in Cesium.js with the declarations in Cesium.d.ts, then review the linked Knockout type definitions for the expected typing. Verify that a TypeScript consumer can import knockout from cesium without @ts-ignore and access its observable functionality type-safely.

Written by the indexing model from the issue text.