Add missing mock method after @types/vscode upgrade

Author: Dave Bartolomeo

extensions/ql-vscode/test/vscode-tests/no-workspace/helpers.test.ts

@@ -272,6 +272,13 @@ describe("helpers", () => {
   class MockEnvironmentVariableCollection
     implements EnvironmentVariableCollection
   {
+    [Symbol.iterator](): Iterator<
+      [variable: string, mutator: EnvironmentVariableMutator],
+      any,
+      undefined
+    > {
+      throw new Error("Method not implemented.");
+    }
     persistent = false;
     replace(_variable: string, _value: string): void {
       throw new Error("Method not implemented.");