Mark most Event properties as readonly

We want to prevent mistaken changes that do something like this:

```ts
foo.onEvent = () => { ... };
```

When they almost always mean:

```ts
foo.onEvent(() => { ... })
```

src/vscode-dts/vscode.proposed.notebookVariableProvider.d.ts

@@ -21,7 +21,7 @@ declare module 'vscode' {
 	}
 
 	export interface NotebookVariableProvider {
-		onDidChangeVariables: Event<NotebookDocument>;
+		readonly onDidChangeVariables: Event<NotebookDocument>;
 
 		/** When parent is undefined, this is requesting global Variables. When a variable is passed, it's requesting child props of that Variable. */
 		provideVariables(notebook: NotebookDocument, parent: Variable | undefined, kind: NotebookVariablesRequestKind, start: number, token: CancellationToken): AsyncIterable<VariablesResult>;