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