feat: Add getScope

[HazAT]

This exposes a new "global" function called getScope

This enables users to do something like this:

Sentry.getScope().setTags({a: 'b'});
Sentry.getScope().setContext('os', {name: 'mac'});

// vs. 

Sentry.configureScope(scope => {
  scope.setTags({a: 'b'});
  scope.setContext('os', {name: 'mac'});
});

It returns an empty Scope in case there is no current hub.

The motivation for this change is:

• Users don't need to fully understand the concept of a scope for the simplest use case there is e.g: setting tags Sentry.getScope().setTags({a: 'b'});
• Exposing all scope functions globally like Sentry.setTag, Sentry.setContext ... increases bundle size significantly and is way harder to maintain since we would always need to sync global API with scope API (thinking about moving towards tracing API), Sentry.getScope() is a good compromise
• This should provide users an easier path to migrate their old SDK codebase to the new one: string replace:

Raven.setTagsContext({a: 'b'}); 
// ->
Sentry.getScope().setTags({a: 'b'});

getScope vs. configureScope

configureScope works differently in a way that in case there is no active client on the hub, the configureScope callback will not be invoked at all, meaning if there are expensive operations this is better than getScope. This is the main difference between these two functions.

[getsentry-bot]

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📖	✅ TSLint passed
📖	@sentry/browser bundle gzip'ed minified size: (ES6: 14.6299 kB) (ES5: 15.3955 kB)

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[dcramer]

We chatted on Slack - plan is to implement the methods directly on the top level class (vs using getScope) and define these as the consumer API for SDKs (so same as Raven -> Sentry.setUser, Sentry.setContext).

[HazAT]

Closing this in favor of #2092