Repository: yt-dlp
Issue: [Niconico]xml danmaku for niconico
Labels: site-enhancement,triage

### DO NOT REMOVE OR SKIP THE ISSUE TEMPLATE

- [X] I understand that I will be **blocked** if I *intentionally* remove or skip any mandatory\* field

### Checklist

- [X] I'm requesting a site-specific feature
- [X] I've verified that I'm running yt-dlp version **2023.03.04** ([update instructions](https://github.com/yt-dlp/yt-dlp#update)) or later (specify commit)
- [X] I've checked that all provided URLs are playable in a browser with the same IP and same login details
- [X] I've searched [known issues](https://github.com/yt-dlp/yt-dlp/issues/3766) and the [bugtracker](https://github.com/yt-dlp/yt-dlp/issues?q=) for similar issues **including closed ones**. DO NOT post duplicates
- [X] I've read the [guidelines for opening an issue](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#opening-an-issue)
- [X] I've read about [sharing account credentials](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#are-you-willing-to-share-account-details-if-needed) and I'm willing to share it if required

### Region

Japan

### Example URLs

https://www.nicovideo.jp/watch/so42039588

### Provide a description that is worded well enough to be understood

Current danmaku output from yt-dlp is in json format and is not compatible with any ass converter I've tried. This https://github.com/AlexAplin/nndownload however is able to output an xml danmaku which can be converted to ass.
I wonder if it is possible to add this feature to yt-dlp.
Below is the files output by yt-dlp and nndownload.
[yt-dlp.zip](https://github.com/yt-dlp/yt-dlp/files/11241619/yt-dlp.zip)
[nndownload.zip](https://github.com/yt-dlp/yt-dlp/files/11241620/nndownload.zip)


### Provide verbose output that clearly demonstrates the problem

- [X] Run **your** yt-dlp command with **-vU** flag added (`yt-dlp -vU <your command line>`)
- [ ] If using API, add `'verbose': True` to `YoutubeDL` params instead
- [X] Copy the WHOLE output (starting with `[debug] Command-line config`) and insert it below

### Complete Verbose Output

```shell
[debug] Command-line config: ['-vU', '--write-subs', '--sub-lang', 'comments', '--sub-format', 'json', '--add-header', 'accept-language:ja', 'https://www.nicovideo.jp/watch/so42039588']
[debug] Portable config "D:\Tools\yt-dlp.conf": ['-N', '8', '--proxy', 'http://127.0.0.1:8188/', '-R', '1000', '--file-access-retries', '10', '--fragment-retries', '1000', '--extractor-retries', '10']
[debug] Encodings: locale cp936, fs utf-8, pref cp936, out utf-8, error utf-8, screen utf-8
[debug] yt-dlp version nightly@2023.04.16.034742 [9874e82b5] (win_exe)
[debug] Python 3.8.10 (CPython AMD64 64bit) - Windows-10-10.0.19045-SP0 (OpenSSL 1.1.1k  25 Mar 2021)
[debug] exe versions: ffmpeg git-2020-05-19-74dfc88, ffprobe git-2020-05-19-74dfc88
[debug] Optional libraries: Cryptodome-3.17, brotli-1.0.9, certifi-2022.12.07, mutagen-1.46.0, sqlite3-2.6.0, websockets-11.0.1
[debug] Proxy map: {'http': 'http://127.0.0.1:8188/', 'https': 'http://127.0.0.1:8188/'}
[debug] Loaded 1797 extractors
[debug] Fetching release info: https://api.github.com/repos/yt-dlp/yt-dlp-nightly-builds/releases/latest
Available version: nightly@2023.04.16.034742, Current version: nightly@2023.04.16.034742
Current Build Hash: c633fda81d539a3e731a6a9145907e0b73beeea81a2f85b8555532e2867dabb3
yt-dlp is up to date (nightly@2023.04.16.034742)
[niconico] Extracting URL: https://www.nicovideo.jp/watch/so42039588
[niconico] so42039588: Downloading webpage
[niconico] so42039588: Downloading comments
[niconico] so42039588: Downloading comments (new)
[info] so42039588: Downloading subtitles: comments
[debug] Formats sorted by: hasvid, ie_pref, lang, quality, res, fps, hdr:12(7), vcodec:vp9.2(10), channels, acodec, filesize, fs_approx, tbr, vbr, abr, asr, proto, vext, aext, hasaud, source, id
[debug] Default format spec: bestvideo*+bestaudio/best
[info] so42039588: Downloading 1 format(s): h264_360p-aac_64kbps-http
Deleting existing file ちびゴジラの逆襲 第1話「怪獣島のちびゴジラ」 [so42039588].comments.json
[info] Writing video subtitles to: ちびゴジラの逆襲 第1話「怪獣島のちびゴジラ」 [so42039588].comments.json
[debug] Invoking niconico_dmc downloader on "niconico_dmc:so42039588/archive_h264_360p/archive_aac_64kbps"
[download] ちびゴジラの逆襲 第1話「怪獣島のちびゴジラ」 [so42039588].mp4 has already been downloaded
[download] 100% of    8.25MiB
```