Support parallel running

[pionxzh]

In the early stage, we actually ran multiple workers at the same time to achieve "parallel running". But it will hit on some confusing errors that the browser might not support running too many web workers at the same time. So it was removed.

We should bring this back with better control over the amount of workers spawned. This should be applied to both the web playground and the CLI.

Concern: the concurrent memory usage will increase and might cause OOM, maybe a CLI option can be a good idea?

Update

CLI has supported --concurrency flag for parallel running.

[StringKe]

Is it possible to properly collect exception error messages and create issues in a semi-automated way?

[pionxzh]

@StringKe Sure, can you help me create an issue for this?

[StringKe]

@StringKe Sure, can you help me create an issue for this?

create by #72