[FEATURE] Automatically handle playlists ordered in reverse

[jmbannon]

Playlists have always been a pain-point with ytdl-sub. If an author adds new videos to the end of a playlist, as opposed to the front, it breaks ytdl-sub's intuition of incremental scraping by breaking on the first (oldest) video. This update now makes it possible to handle this in the prebuilt TV Show presets:

• Add each URL variable (url, url2, ...) into the download portion of the subscription twice
• First definition is what we all know and use, simply scrapes first-to-last, then downloads last-to-first
• Second definition does the following:
  • Check to see if a URL is a YouTube playlist URL, if so...
  • Set the field to download, but with modifications to scrape last-to-first, then download first-to-last
  • Otherwise...
  • Set the field to an empty string, which means ytdl-sub will skip it

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.12403% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 97.02%. Comparing base (083db0d) to head (cf12c36).

Files	Patch %	Lines
src/ytdl_sub/downloaders/url/downloader.py	90.00%	4 Missing :warning:
src/ytdl_sub/entries/script/variable_types.py	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #948 +/- ## ========================================== - Coverage 97.02% 97.02% -0.01% ========================================== Files 115 115 Lines 7339 7394 +55 ========================================== + Hits 7121 7174 +53 - Misses 218 220 +2 ```

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