`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
