[tiktok] Add new extractor

[Flat]

Please follow the guide below

• 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])
• Use Preview tab to see how your pull request will actually look like

---

Before submitting a pull request make sure you have:

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

• 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
• New extractor
• New feature

---

Description of your pull request and other information

Provides an extractor for ((v)m).tiktok.com closes #18108

[dstftw]

Read coding conventions and fix code.

[Flat]

I believe you were referring to optional fields being fatal (using an index which may not exist). The latest commit should remedy that. Let me know if I missed anything else.

[dstftw]

Nothing conceptually changed.

[Flat]

I hope this is now more in-line with the code conventions.

[Flat]

I've added all available formats and get the extension in a similar fashion to https://github.com/rg3/youtube-dl/blob/c3bcd206eb031de30179c88ac7acd806a477ceae/youtube_dl/extractor/hearthisat.py#L106-L111 and https://github.com/rg3/youtube-dl/blob/c3bcd206eb031de30179c88ac7acd806a477ceae/youtube_dl/extractor/wdr.py#L75-L78 as the downloads URLs do not provide a file extension.

[Flat]

Sorry it took so long to get into a semi-mergable state. Thank you for the reviews and ultimately implementing and improving the extractor. Cheers.