On_ready callback for setup_dap_main_class_configs()

[mikehaertl]

I wonder if it would make sense to be able to pass a callback to setup_dap_main_class_configs(). It would be called when the configuration is ready.

I may do things wrong but right now to start debugging I do:

• Call setup_dap_main_class_configs() or :JdtRefreshDebugConfigs
• Wait ...
• Call require('dap').continue() to start debugging

I can't put both into a function because continue() has to wait until the configuration is ready.

With a callback this could be streamlined to:

require('jdtls').setup_dap_main_class_configs({
  on_ready = function(config)
    require('dap').continue()
  end
})

[mfussenegger]

Makes sense to me. Opened up #419