feat(jest-runtime): expose isTornDown variable

This commit adds a new feature to the Jest runtime package that exposes
the `isTornDown` variable as a way to check if the Jest environment has
been torn down programmatically. The `isTornDown` variable is a
read-only boolean that is set to `true` when the environment is torn
down and `false` otherwise.

The commit also adds documentation for the `isTornDown` variable to the
Jest runtime module.

Closes jestjs#13640

packages/jest-environment/src/index.ts

@@ -172,6 +172,15 @@ export interface Jest {
    * local module state doesn't conflict between tests.
    */
   isolateModules(fn: () => void): Jest;
+  /**
+   * A flag that indicates whether the Jest environment has been torn down.
+   * @example
+   * if (jest.isTornDown) {
+   *   // The Jest environment has been torn down, so stop doing work
+   *   return;
+   * }
+   */
+  isTornDown: boolean;
   /**
    * Mocks a module with an auto-mocked version when it is being required.
    */

packages/jest-runtime/src/index.ts

@@ -2225,6 +2225,7 @@ export default class Runtime {
       },
       getTimerCount: () => _getFakeTimers().getTimerCount(),
       isMockFunction: this._moduleMocker.isMockFunction,
+      isTornDown: this.isTornDown,
       isolateModules,
       mock,
       mocked,