Added downloader for YT-DLP, and partially fixed tests

[micah686]

Removed the powershell script which was broken and would download the files into the wrong folders. Changed the tool to yt-dlp instead of youtube-dl Added two functions to get platform-dependent yt-dlp and ffmpeg binaries. Added a github workflows to automatically build and test the tool.

In tests, DownloadTests needs to be modified, because they are failing with this error: Assert.AreEqual failed. Expected:<>. Actual:<WARNING: External Downloader arguments given without specifying name. The arguments will be given to all external downloaders>.