This is obsoleted by the standard semanticTokens request family.
As well as the protocol details, this allows us to remove a bunch of plumbing
around pushing highlights to clients.
This should not land until the new protocol has feature parity, see D77702.
clang-tidy: error: no member named 'TheiaSemanticHighlighting' in 'clang::clangd::ClangdLSPServer::Options' [clang-diagnostic-error]
not useful