Move PimCommon::SieveSyntaxHighlighterUtil to KSieveUi
Summary:
As only user of PimCommon::SieveSyntaxHighlighterUtil is KSieveUi as it
is only used for tests, it is save to move it there.
Test Plan: compile & runTests
Reviewers: KDE PIM, mlaurent, vkrause
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D12409