[KuitFormatterPrivate] Start porting QRegExp to QRegularExpression

Summary:
Port toVisualText() and finalizeVisualText() to use QRegularExpression.
Replace ENTITY_SUBRX macro with a static const char*.

Test Plan: make && ctest

Reviewers: Frameworks, ilic, dfaure, mlaurent, aacid

Reviewed By: dfaure

Subscribers: kde-frameworks-devel, ltoscano

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26285