[extractor/discogs] Add extractor

[rjy]

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

Add an extractor for the playlists embedded in Release and Master Release pages on discogs.com (an online music database). The generic extractor used to be able to read these, but that stopped working a long time ago.

Example URLs:

• https://www.discogs.com/release/1-The-Persuader-Stockholm
• https://www.discogs.com/master/113-Vince-Watson-Moments-In-Time

Template

Before submitting a pull request make sure you have:

• At least skimmed through contributing guidelines including yt-dlp coding conventions
• Searched the bugtracker for similar pull requests
• Checked the code with flake8 and ran relevant tests

In order to be accepted and merged into yt-dlp each piece of code must be in public domain or released under Unlicense. Check all of the following options that apply:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Fix or improvement to an extractor (Make sure to add/update tests)
• New extractor (Piracy websites will not be accepted)
• Core bug fix/improvement
• New feature (It is strongly recommended to open an issue first)