Add eslint rule for potentially unsafe disposable patterns

[mjbvz]

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:

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