This PR adds a new feature to the Jest runtime package that exposes the isEnvironmentTornDown method to check if the Jest environment has been torn down programmatically. The isEnvironmentTornDown function returns a 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 #13640

Summary

The motivation for exposing the isTornDown variable in the Jest runtime package is to provide a way for tests to check if the Jest environment has been torn down programmatically.
issue #13640 requesting this feature.

Test plan

Test Objective

The objective of this test is to verify that the isTornDown variable is correctly exposed in the Jest runtime package and that it can be used to check if the Jest environment has been torn down programmatically.

Test Steps

1. Clone the Jest repository and make sure that the isEnvironmentTornDown feature has been implemented and is ready for testing.
2. Create a new test file that imports the jest global object and uses the isEnvironmentTornDown method to check if the Jest environment has been torn down.
3. Run the test file using the jest command.
4. Verify that the test file runs successfully and that the isTornDown variable is correctly set to false when the Jest environment is not torn down.
5. Modify the test file to tear down the Jest environment after the test has run.
6. Run the test file again using the jest command.
7. Verify that the test file runs successfully and that the isTornDown variable is correctly set to true when the Jest environment has been torn down.
8. Modify the test file to check the isEnvironmentTornDown method in a loop that advances the fake timers by a specified amount of time.
9. Run the test file again using the jest command and verify that the test file stops executing when the Jest environment is torn down.
10. Modify the test file to check the isEnvironmentTornDown method in a loop that does a lot of work and can time out.
11. Run the test file with a long timeout value (e.g. --timeout=10000) and verify that the test file stops executing when the Jest environment is torn down or the test times out.

Test Expected Results

• The test file should run successfully and the isTornDown variable should be correctly set to false when the Jest environment is not torn down.
• The test file should run successfully and the isTornDown variable should be correctly set to true when the Jest environment is torn down.
• The test file should stop executing when the Jest environment is torn down or the test times out.