Fix skipOffset with dynamic RegExpr

Summary:
The pattern used depends on the previous contexts so skipOffset cannot be computed with a dynamic regex.

BUG: 399388

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D24179