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