This epic is the outcome of the investigation in https://github.com/eclipse/che/issues/4609. The goal is to allow multiple language servers to be registered for the same type of file (by extension or by path regex) and to merge the results according to the policies set out here: https://code.visualstudio.com/docs/extensionAPI/vscode-api#_languages
The immediate need for this comes from the bayesian language server, which would like to add diagnostic information to files like pom.xml or package.json. Also, a number of linter plugins for VSCode are implemented as language servers (https://github.com/Microsoft/language-server-protocol/issues/8).