feat: Use `declare class` for namespaced types

[ExE-Boss]

Unlike https://github.com/microsoft/TypeScript-DOM-lib-generator/pull/858, which failed because classes declared in the global scope exist only on the global Declarative Environment Record, classes exported from TypeScript’s namespace construct exist as own properties of the namespace object, which matches the WebIDL behaviour.

To consider:

• [ ] Emit attributes as getter‑setter pairs to match runtime behaviour.

Depends on:

• [x] ~~https://github.com/microsoft/TypeScript-DOM-lib-generator/pull/962~~ (merged)

[sandersn]

This doesn't address https://github.com/microsoft/TypeScript-DOM-lib-generator/pull/858#issuecomment-644294982, does it? That should be the first step in this process.