Adds loadstate tracking to frame

[dbrody]

Allows for listening to loadstate changes on the page to wait for things like networkidle, domcontentloaded and load etc.

page |> Playwright.Page.on("loadstate", fn frame, event ->
  if Enum.member?(frame.loadstate, "networkidle") do
     # Network Idle
  end
end)

Also adds response to the requestFinished event.

[dbrody]

Closing as I think this is stale given all the changes and additions to the repo.