Update worker.ex w/ placeholders

lib/playwright/worker.ex

@@ -4,7 +4,21 @@ defmodule Playwright.Worker do
   """
   use Playwright.ChannelOwner
 
+  # @spec evaluate(Worker.t(), function() | binary(), EvaluationArgument.t()) :: Serializable.t()
+  # def evaluate(worker, page_function, arg \\ nil)
+
+  # @spec evaluate_handle(Worker.t(), function() | binary(), EvaluationArgument.t()) :: JSHandle.t()
+  # def evaluate_handle(worker, page_function, arg \\ nil)
+
   # @spec expect_event(t(), binary(), function(), options()) :: map()
   # def expect_event(worker, event, predicate \\ nil, options \\ %{})
   # ...delegate wait_for_event -> expect_event
+
+  # on(...):
+  #   - close
+  # @spec on(t(), binary(), function()) :: nil
+  # def on(worker, event, callback)
+
+  # @spec url(Worker.t()) :: binary()
+  # def url(worker)
 end