[ninjastream] Add new extractor

[ProgrammingByPermutation]

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Before submitting a pull request make sure you have:

• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• 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
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

Adding a new extractor for the https://ninjastream.to website.

[dstftw]

Read coding conventions.

[ProgrammingByPermutation]

Read coding conventions.

@dstftw Can you please confirm if this addresses the coding convention issues?

[cypheron]

@ProgrammingByPermutation again check #27691 - imho no point in opening PR's in this project as long as they don't get responsible maintainers.

[bersbersbers]

I installed from 5adc36f, but I am getting

WARNING: unable to extract file-watch-jwplayer

This seems to be addressed in #28928, which is working fine. Mentioning this here for anyone stumbling across this PR in #28054 (which could be updated).