feat(api): Add top-level API for getting scopes

[sentrivana]

Adding get_current_scope(), get_isolation_scope() and get_global_scope() to the top-level API to make them easier to use. It'll also simplify our migration guide(s).

It's also part of the RFC and JS has it.

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[sentrivana]

Shelving this for now to keep the top level API minimal. This is something only advanced users will need to use. We can reevaluate at a later point if this should be added to the top-level API.