This patch:
- adds FFMPEG binaries to the NPM packages
- adds a screencast test to make sure that screencast works. This currently relies on private screencast APIs.
NOTE: with this patch playwright package size grows from `650KB` to `4.2MB`.
This makes it easier to reason about our packages.
The only difference is what each package downloads.
When the browser is not downloaded, it will fail to launch.
Each browser gets a 'download' attribute in the browser.json file.
