[v8] Ensure Loader Script remains fully compatible

[mydea]

We need to ensure that the loader script remains as much compatible as possible... So users that get the latest CDN bundle for the Loader will not suddenly have broken websites.

This includes a compatibility layer for:

• configureScope() (root method for the loader)
• class integrations (??)
• what about other, general APIs? Most prominent would be startTransaction()...

[Lms24]

@mydea Can we close this issue in favour of #12187? Given we intentionally held off from adding v8 to the loader at the moment, I think we're fine on this end, no?

[mydea]

Yes!