Feature request: support zref and zref-clever

[qcts33]

First of all, thanks for this excellent work! It would be nice to support the zref and zref-clever package as an alternative reference system. The zref package provides the extensible referencing system with the \zlabel command. The zref-clever package is based on zref and provides the same functions as the old cleveref package, but using the \zcref command.

[DasNaCl]

This is somewhat supported already by extending the texlab.settings.experimental.labelReferenceCommands and texlab.settings.experimental.labelDefinitionCommands options in the language server setup. It is worth noting that the zref and zref-clever package documentation says that those packages are to be considered experimental themselves. Nevertheless, supporting the label definition and reference commands for these packages by default should be a relatively easy addition to the code. I'll add a pullrequest for making them default, but I'm unsure if they should be default or not. (or if my PR is missing commands)