[Schooltube] Add new extractor

[flomk]

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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

Added a new extractor for education website schooltube.com - issue #13091.
Currently only works on single videos and doesn't work on channel page but I am working on adding support for that.

[flomk]

Why was this closed?
Never mind I see, will fix.

[dstftw]

It's covered by generic extractor.

[flomk]

Yes but the person who created the issue wanted more metadata such as keywords than is provided by the generic extractor.

[dstftw]

No such meta field keywords is recognized.

[flomk]

I didn't realize, my apologies.

[wertercatt]

keywords in this context is equivalent to the meta field tags