[extensions] extension commands no longer work

[JanKoehnlein]

Extension commands (commands defined in the config fiels,implemented in a jar in the WS and dynamically added by the server) no longer get an entry in the menu.

The reason is that Theia/Monaco LC switched to a global command registry, and our hooks that atuomatically add labels (prerequisite to be added in the quick command list) and menu entries are no longer called.

We may have to add a hook in Theia that allows extensions to hook into the registration of server-side commands https://github.com/Microsoft/vscode-languageserver-node/blob/7850be5cce4ed39485150969de14947d08c47375/client/src/client.ts#L2200
That is unfortunately a bit complicated, as the ExecuteCommandFeature is not exported by vscode. @akosyakov filed a PR for that microsoft/vscode-languageserver-node#379