Migrate from the direct runtime access throw the go routines

[olegbespalov]

What?

During the #67 (comment) we figured out that it's essential to not have access to the goja runtime inside the go routines because that could lead to the race.

There are places in the module where we have such access; we should migrate them accordingly:

• https://github.com/grafana/xk6-webcrypto/blob/main/webcrypto/subtle_crypto.go#L193
• https://github.com/grafana/xk6-webcrypto/blob/main/webcrypto/subtle_crypto.go#L293

Why?

Having access to the throw routine could lead to the race.