BrightScript: Add workaround for QtCreator 'endsub' highlighting

Summary:
QtCreator requires that keyword should be wrapped with white spaces on
both ends. Kate requires only trailing white space after keyword. This
patch replaces detection of 'endsub' and 'endfunction' with
StringDetect instead of keyword to satisfy both.

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D17522