Move PimCommon::SieveSyntaxHighlighterUtil to ksieveui
Summary:
As only user of PimCommon::SieveSyntaxHighlighterUtil and only for
tests, it is save to move it in here.
Test Plan: compile & runTests
Reviewers: KDE PIM, mlaurent, vkrause
Reviewed By: KDE PIM, mlaurent
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D12401