SCSS: fix interpolation (#{...}) and add the Interpolation color

Summary: Only interpolation with variable (#{$...}) and beginning of the string ("${...}...") was highlighted.

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D23195