Add syntax highlighting for Solidity

Summary:
FEATURE: 411867

The XML file was created by Robert Kaiser and attached in: https://bugs.kde.org/show_bug.cgi?id=411867

I took the test file from the Solidity documentation: https://solidity.readthedocs.io/en/latest/solidity-by-example.html

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/D27313