Extension `classpath` is currently only allowed to be relative to the project workspace

[esmasth]

It is beneficial to have some centrally deployed extension jar files across projects. A start can be to allow for absolute classpaths outside the project which may be referenced via the user level yang.settings.

This can also be an opportunity to consider other mechanisms to initialize YANG LSP server which are in server or global context and not only user/workspace/project context. Using workspace/didChangeConfiguration or the newer workspace/getConfiguration method can be considered.