Fix interaction mode for non-read-only Expressions editor.

[biagas]

Description

Resolves #19866

Changed the non-read-only TextInteractionFlags to Qt::TextEditInteraction. Qt docs state this flag is the default for a text editor and is the same as:

TextSelectableByMouse | TextSelectableByKeyboard | TextEditable

Type of change

• [X] Bug fix~~
• [ ] New feature
• [ ] Documentation update
• [ ] Other

How Has This Been Tested?

I tried using arrow keys to move around, and mouse to select text with success.

Checklist:

• [X] I have commented my code where applicable.
• [ ] I have updated the release notes.
• [ ] I have made corresponding changes to the documentation.
• [ ] I have added debugging support to my changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have confirmed new and existing unit tests pass locally with my changes.
• [ ] I have added new baselines for any new tests to the repo.
• [X] I have NOT made any changes to [protocol or public interfaces][3] in an RC branch.