FlatBuffers schema syntax highlighting

Summary:
Support for FlatBuffers schema, for grammar see
https://google.github.io/flatbuffers/flatbuffers_grammar.html

Test Plan: Opened some Flatbuffers in kate and Qt Creator

Reviewers: dhaumann, cullmann

Reviewed By: dhaumann, cullmann

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

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D18864