Update Hub & Scope handling

[mydea]

We need to implement getsentry/rfcs#122.

### Tasks
- [x] Implement `getCurrentScope()`
- [x] Implement experimentally https://github.com/getsentry/sentry-javascript/pull/9799
- [ ] https://github.com/getsentry/sentry-javascript/pull/9920
- [x] Implement `getIsolationScope()`
- [ ] https://github.com/getsentry/sentry-javascript/pull/9804
- [ ] https://github.com/getsentry/sentry-javascript/pull/9887
- [ ] https://github.com/getsentry/sentry-javascript/pull/9890
- [x] Deprecate `getCurrentHub()`