[clangd] Support new semanticTokens request from LSP 3.16.

Summary:
This is a simpler request/response protocol.

Reference: https://github.com/microsoft/vscode-languageserver-node/blob/master/protocol/src/protocol.semanticTokens.proposed.ts

No attempt to support incremental formatting (yet).

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76663