Add eslint rule for potentially unsafe disposable patterns (#209555)

`DisposableStore`/`MutableDisposable` properties should almost always be `readonly`. Otherwise it's easy to accidentally overwrite the property and leak the previous value. This commonly happens with code such as:

```ts
class Foo {
     private disposables = new DisposableStore();

     bar() {
         this.disposables = new DisposableStore(); // leaks old values
         ...
     }
```

This change adds an eslint rule to enforce this and adopts `readonly` for the caught cases. I only needed to add 2 suppression comments, which seems like an acceptable tradeoff for helping catch a common mistake

src/vs/workbench/contrib/notebook/browser/viewModel/notebookViewModelImpl.ts

@@ -102,7 +102,7 @@ export interface NotebookViewModelOptions {
 }
 
 export class NotebookViewModel extends Disposable implements EditorFoldingStateDelegate, INotebookViewModel {
-	private _localStore: DisposableStore = this._register(new DisposableStore());
+	private readonly _localStore = this._register(new DisposableStore());
 	private _handleToViewCellMapping = new Map<number, CellViewModel>();
 	get options(): NotebookViewModelOptions { return this._options; }
 	private readonly _onDidChangeOptions = this._register(new Emitter<void>());