Lua: fix multi-line string

Summary:
Strings that end with \z or \ can continue to the next line

BUG: 395515

Reviewers: dhaumann, cullmann, #ktexteditor, #kate

Reviewed By: dhaumann, #ktexteditor, #kate

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D13829