Add Are.na channel extractor (resolves #5847)

[dev-KingMaster]

Summary

• Adds support for downloading media from Are.na channel pages.
• New extractor: gallery_dl/extractor/arena.py (line 1)
• Supports URLs like https://are.na//
• Closes [Site Support] are.na #5847

Details

• API usage:
  • Channel metadata: GET https://api.are.na/v2/channels/{slug}
  • Channel contents: GET https://api.are.na/v2/channels/{slug}/contents?page=&per=100
• Extracts:
  • Images via image.original.url (fallback to display/large when needed)
  • File attachments via attachment.url
  • Adds source_url when present (for Link blocks)
• Skips non-downloadable items (e.g., nested channels without media).
• Populates directory/filename metadata using GalleryExtractor defaults (includes channel slug/id, title, numbering).
• Honors extractor.page-reverse.

URL Patterns

• Channel: https://are.na//

Testing

• Validated against public channels (downloaded first 5 items):
  • https://are.na/evan-collins-1522646491/cassette-futurism
  • https://are.na/lachie/transparent-tech-cobxde9pu40
• Verified both URL generation (-g) and file downloads.

Contribution by Gittensor, learn more at https://gittensor.io/

[dev-KingMaster]

Hi @mikf Could you review this pr and leave any comments? Thanks