Fix incorrect regexs for string in backend highlighters

Summary: Now string highlighter regex highlighte two strings, standing neary, as one string, that incorrect. So it fix it.

Test Plan:
1) Open Octave, Scilab, Lua, Sage session, write `'3', '4', '5'`
2) Make sure, that string highlighting wrong.
3) Appy the patch
4) Test again and check, that all okay

Reviewers: #cantor, asemke

Subscribers: #cantor, #kde_edu

Tags: #kde_edu, #cantor

Differential Revision: https://phabricator.kde.org/D12052