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/api/common/extHostComments.ts

@@ -397,7 +397,7 @@ export function createExtHostComments(mainContext: IMainContext, commands: ExtHo
 
 		private _commentsMap: Map<vscode.Comment, number> = new Map<vscode.Comment, number>();
 
-		private _acceptInputDisposables = new MutableDisposable<DisposableStore>();
+		private readonly _acceptInputDisposables = new MutableDisposable<DisposableStore>();
 
 		readonly value: vscode.CommentThread2;