Introduce a bot to test downstream

[magic-akari]

It's better to have a bot like @typescript-bot with test top100 supports.

[kdy1]

I have thought about this, but not sure about some steps.

1. Search npm, finding packages with @swc/core as a dev dep.
2. Ignore if the repository field in package.json does not exists
3. Using the repository field, clone the repository.

But then how can we know the way to run tests?