[request] generic `Property` TS type

I suggest that adding a type parameter to Property class type in `Cesium.d.ts`:

```ts
export class Property {
constructor();
readonly isConstant: boolean;
readonly definitionChanged: Event;
getValue(time: JulianDate, result?: T): T;
equals(other?: Property): boolean;
}
```

Why:

Currently Property's value is any, so it is loose. We can use the types to more tightly constrain the content of a Property.

Notes:

- I know type definitions are generated from JSDocs, but I don't know how to represent generic type in JSDoc...
- Setting the default type parameter to `any` is good idea because it will prevent to compromise compatibility.

Start with the Property declaration in Cesium.d.ts and trace how its types are generated from JSDoc. Determine how a generic type parameter can be represented while preserving the default any behavior, then verify that the generated declaration exposes the requested getValue and equals signatures without breaking compatibility.

Written by the indexing model from the issue text.