Make decoration provider API public

Make the following API public

```ts
export interface DecorationProvider {
	onDidChangeDecorations: Event<undefined | Uri | Uri[]>;
	provideDecoration(uri: Uri, token: CancellationToken): ProviderResult<DecorationData>;
}

export namespace window {
	export function registerDecorationProvider(provider: DecorationProvider): Disposable;
}
```