Manage langserver installation and updates

[jryans]

It would nice to help the user by (with their consent) managing installation and updating of the langserver.

For new users, offering to auto-install the langserver means one less manual step.
For existing users, we can check whether the installed langserver matches the latest version, and if not, offer to update it.

Since a large chunk of extension functionality is actually in the langserver, it's particularly important to keep it updated.