Consider supporting proposed semanticHighlight notification

[ebkalderon]

The textDocument/semanticHighlight server-to-client notification is a proposed addition to the Language Server Protocol which would allow servers to perform syntax highlighting of open text documents (upstream issue, prototype JS implementation, relevant lsp-types PR).

Since this is a proposed protocol feature currently not in the specification yet, we might need to gate it with a "proposed" feature flag, similar to what lsp-types does for the feature currently.

This feature is fairly low priority, possibly below #145, but would nonetheless be nice to have.

[oovm]

I also need this, can we add the "proposed" feature?

[ebkalderon]

Resolved in #270 under the "semantic tokens" feature of LSP 3.16! 🎉