Add syntax definition for Elm language to syntax-highlighting

Summary: This commit adds syntax definition for Elm language to syntax-highlighting. This commit adds syntax definition for Elm language to KDE/syntax-highlighting. Elm is a purely functional language of ML family that compiles to Javascript for creating web browser-based graphical user interfaces.

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, dhaumann

Subscribers: nibags, dhaumann, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D19438