[ie/Instagram] Support new reels URL format

[amir16yp]

Fixes /reels/ prefixed url not working in the instagram extractor

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

Fixes instagram /reels/ url not being detected and falling back to the generic extractor. Closes #6689

Template

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• At least skimmed through contributing guidelines including yt-dlp coding conventions
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• Checked the code with flake8 and ran relevant tests

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