Add support for file downloads

[tylerhunt]

Add PlaywrightEx.Download and PlaywrightEx.DownloadWaiter to handle file downloads.

Example:

{:ok, download_ref} = Page.expect_download(page.guid, timeout: @timeout)
{:ok, _} = Frame.click(frame.guid, selector: Selector.link("Download"), timeout: @timeout)
{:ok, path} = Page.await_download(download_ref)