Use new pull/push model for configuration sync at startup

[bmeneg]

The way the configuration is being sync'ed at startup in this Perl Language Sercer seems to be deprecated for quite some time already and might get removed from vscode-languageclient code any time.

To avoid surprises it would be a good time to change from "client notification" (LanguageClientOptions vscode client code) to "server request" using workspace/configuration request and keep it up-to-date with didChangeConfiguration (already implemented).