This is a Haxe library with type definitions for the Language Server Protocol. Essentially it's a Haxe port of the vscode-languageserver-protocol / vscode-languageserver-types libraries from vscode-languageserver-node. It depends on vscode-json-rpc.
It's used by the Haxe Language Server and was separated from it so one could implement their own client and/or server using Haxe.