Add support for semantic highlighting (LSP 3.16)

[alanz]

See emacs-lsp/lsp-mode#1482 where they discuss adding support in emacs lsp-mode. Which means vscode and lsp-mode will be supporting this shortly.

[jneira]

Yeah, it is included in the incoming 3.16 spec version: https://microsoft.github.io/language-server-protocol/specifications/specification-3-16/#version_3_16_0
I've added it in the general issue about the spec: #134

[jneira]

Closed by #314