[clangd] Added the vscode SemanticHighlighting feature code but did not enable it in the client.

Summary: Added the code for the StaticFeature that must be registered to the client. Also decoding the notification data into objects. Did not register it to the client yet.

Reviewers: hokein, ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65998

llvm-svn: 368568