Fix: Email highlighting for unclosed parenthesis in Subject header

Summary:
Unfortunately, the "Subject:" header needs to be treated independently
of the other headers, since it may contain arbitrary text such as paranthesis
that is not a comment or similar. The fix is to add another context without
the comment highlighting.

BUG: 398717

Test Plan: make test

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: #kate, #frameworks

Differential Revision: https://phabricator.kde.org/D15556