Add AtCoder support

Hi,

TLE is cool!

However, I ran out of `;vc +div4` so I want to do something like `;vp +abc` (`vp`="Virtual Participation", `abc`="AtCoder Beginner Contest").

Would it be a good idea to incorporate AtCoder support in TLE, or is it out of scope?

Also, do you know of any existing solutions for this exact problem?

Currently, I have the following (semi-correct) [script](https://github.com/osalbahr/TLE/blob/master/atcoder.py) that I keep running until I get a valid contest:

```py
#!/usr/bin/env python3

import random
import requests

nums = [f"{n:03}" for n in range(1, 1000)]

while True:
idx = random.randint(0, len(nums) - 1)

url = f"https://atcoder.jp/contests/abc{nums[idx]}"

if response := requests.get(url):
print(url)
break

print(f"Error requesting {url}:", response)
ans = input("Re-run the script? (y/N) ").lower()

if not ans or ans[0] != "y":
break
```

It works fine for now, but it will get annoying at some point.

I also just want to practice solving a "real problem".

Start by reading the linked atcoder.py script and the issue’s proposed `;vp +abc` interaction. Determine how AtCoder contest discovery would fit into TLE, then verify that the requested virtual-participation flow works for AtCoder Beginner Contests and handles unavailable contests without repeated manual retries.

Written by the indexing model from the issue text.