Make semantic highlighting available via LSP

[numirias]

Let's investigate if semantic highlighting can be provided via LSP any time soon.

• LSP implementations for neovim are being actively discussed (neovim/neovim#5522) and clients exist (e.g. LanguageClient-neovim).
• An extension of LSP for semantic highlighting is also being discussed (Microsoft/language-server-protocol/issues/18).
• However, the latest protocol implementation proposal (Microsoft/language-server-protocol/pull/124) seems a bit too limited. While we could work with the provided IDs and modifiers for coloring, we couldn't port features like highlighting related nodes at the cursor position. (Also, how would that affect performance?)
• It will probably take quite some time from the protocol specification to the implementation in a neovim LSP client.

[svenefftinge]

We started working on a proposal : https://github.com/Microsoft/vscode-languageserver-node/pull/367 Please provide feedback.