Use app shell methods for expand / collapse for toggle terminal command

[bryanchenmchp]

What it does

Fixes the toggleTerminal() method to use the app shell expand/collapse methods.

Resolves #13129

How to test

1. Run yarn electron build and then yarn electron start
2. Press ctrl+` to trigger the "Terminal: Toggle Terminal" command
3. The bottom panel should animate showing / hiding, this didn't happen before because the bottom panel was directly being shown/hidden

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[tortmayr]

LGTM and works as expected! Thanks!

[tortmayr]

@bryanchenmchp There is a conflict in the changelog that needs to be resolved before we can merge this

[bryanchenmchp]

Updated changelog