[Gab] Add new extractor

[runraid]

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• 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])
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Before submitting a pull request make sure you have:

• [ x] At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
• [ x] Searched the bugtracker for similar pull requests
• [x ] 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:

• [x ] 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
• [x ] New extractor
• New feature

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Description of your pull request and other information

This PR adds a new extractor to extract videos from https://gab.com/

#18226 requested it.

[selfisekai]

• gab social is an open source, selfhostable and federated platform, the extractor should not be limited to just gab.com
• gab social is a fork of mastodon, extractor should be renamed from gab to mastodon and be adjusted to work with tootsuite/mastodon
• both mastodon and gab offer an open JSON REST API, maybe consider using it instead? https://docs.joinmastodon.org/methods/statuses/

[runraid]

No. You do it. I'll close this PR.

[selfisekai]

XDDDDDDDDDDDDDDDDDDDDDDDDDDDD