Change startDebugging to connect to parent adapter if type=server with executable

[mfussenegger]

With this change startDebugging should work out of the box for debug
adapters configured with type=server and an executable.

For example, for vscode-js-debug, instead of a custom adapter factory
like:

require("dap").adapters["pwa-node"] = function(on_config, config, parent)
  local target = config["__pendingTargetId"]
  if target and parent then
    local adapter = parent.adapter --[[@as ServerAdapter]]
    on_config({
      type = "server",
      host = "localhost",
      port = adapter.port
    })
  else
    on_config({
      type = "server",
      host = "localhost",
      port = "${port}",
      executable = {
        command = "node",
        args = {"/path/to/js-debug/src/dapDebugServer.js", "${port}"},
      }
    })
  end
end

It will be possible to use the simpler definition:

require("dap").adapters["pwa-node"] = {
  type = "server",
  host = "localhost",
  port = "${port}",
  executable = {
    command = "node",
    args = {"/path/to/js-debug/src/dapDebugServer.js", "${port}"},
  }
}