[youtube] squarespace embeds (youtube-dl issue #21294)

[galgeek]

Please follow the guide below

• You will be asked some questions, please read them carefully and answer honestly
• Put an x into all the boxes [ ] relevant to your pull request (like that [x])
• Use Preview tab to see how your pull request will actually look like

---

Before submitting a pull request make sure you have:

• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
• Searched the bugtracker for similar pull requests
• Checked the code with flake8

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• 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?

• Bug fix
• Improvement
• New extractor
• New feature

---

Description of your pull request and other information

This PR adds support for recent youtube embeds on squarespace sites—see #21294

[dstftw]

This does not scale since any video can be embedded this way not only Youtube.
What should be done is unescaping and expanding embed code in place in generic extractor. This will allow supporting any video embedded this way.

[galgeek]

Thanks, @dstftw! I agree, something like what you're suggesting is a better fix.

#21859 is more like what you're thinking of, I think...