[TikTok] Add support

[CasualYT31]

This PR adds support for TikTok slideshows.

List of supported URL formats:

• /photo/
• /video/
• vm.tiktok.com
• vt.tiktok.com
• /t/
• User profiles (tiktok.com/@username)

I haven't performed extensive testing yet so there could well be edge cases that aren't covered by the extractor as it stands right now. I have tested with cookies, however, and I am able to extract photos from private accounts (as well as detect when cookies are required to extract photo URLs).

Closes:
#3061
#4177

[CasualYT31]

Before merging, I'd like to try supporting user profile scraping.

I'll likely use yt-dlp to gather all of the links up and then feed them into the base extractor using urls(). Ideally I would replicate the logic in yt-dlp instead but it's quite a bit more work, plus I don't think it's unreasonable to make yt-dlp a requirement for this one feature seeing as users are likely to have it installed already.

I'll introduce a new tiktok-range option that can be used to prevent scraping more pages than necessary for huge accounts. This differs from the ordinary --range option as that dictates which extracted links to download, whereas -o tiktok-range will tell the TikTok extractor which links to extract.

I'll also think about supporting the download of profile pictures.

Once I've completed this, or decided that it's out of the scope of this PR, I'd say it's ready for merging, barring any additional feedback 👍

[CasualYT31]

I should admit that user profile extraction can be a tiny bit flakey. Very occasionally yt-dlp will fail to extract user profile information and there's not a lot gallery-dl can do about it. Is it okay to keep these tests in, or should they be skipped?