Add scala.lang for Scala syntax highlighting

This file originated in the scala-dist project:

https://github.com/scala/scala-dist/tree/master/tool-support/src/gedit

It was written by

Min Huang <min.huang@alumni.usc.edu>
Felix Dietze <felix@beerleader.net>

with small fixes by

Anthony Kirby <anthony@anthony.org>
Adam Dingle <adam@yorba.org>

Originally the file was licensed under the Scala license:

https://github.com/scala/scala/blob/master/docs/LICENSE

All authors above have granted permission for relicensing under
the LGPL 2.1 or higher for inclusion in gtksourceview.