PR: Wait on all shutdown threads (IPython console)

[impact27]

Description of Changes

Make sure spyder doesn't quit while some threads are running, leading to a crash

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

• Included a screenshot or animation (if affecting the UI, see Licecap)

Issue(s) Resolved

Fixes #20561

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

I certify the above statement is true and correct:

[ccordoba12]

Thanks @impact27 for your help with this! Small suggestion for you, otherwise looks good to me.

[ccordoba12]

Thanks @impact27!