get children when node expands (#201909)

* get children when node expands

* switch to asynchronous tree for more natural lazy loading

* store variables by ID so we can look up the correct instance

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

@@ -16,7 +16,7 @@ declare module 'vscode' {
 
 	interface VariablesResult {
 		variable: Variable;
-		namedChildrenCount: number;
+		hasNamedChildren: boolean;
 		indexedChildrenCount: number;
 	}