More porting from QRegExp to QRegularExpression

Summary: Less deprecated QRegExp usage.

Test Plan:

• KTextEditor::Commands tested, still work
• make test is still green

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D23385