[extractor/s4c]: Add extractor

[ifan-t]

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

This extractor adds support for videos on https://www.s4c.cymru/clic which hosts videos from the Welsh language S4C TV channel.

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)

Copilot Summary

🤖 Generated by Copilot at 076d8fd

Summary

✨📺🐉

Add support for downloading videos from S4C, a Welsh language broadcaster. Create a new s4c.py module that defines the S4CIE extractor class and import it in _extractors.py.

S4CIE extracts
videos from Welsh website
autumn leaves fall fast

Walkthrough

• Add support for S4C video extraction (link, link)
  • Import S4CIE class from s4c.py module in _extractors.py (link)
  • Define S4CIE class in s4c.py module that inherits from InfoExtractor (link)