feat(core): Move globals to __SENTRY__ singleton

[timfish]

This PR moves globalScope and globalMetricsAggregators to the __SENTRY__ global singleton.

This means that if the files that these globals are in get bundled or different versions of @sentry/core are used, we can be sure that they'll still access the same globals.

[mydea]

This is good, I also wanted to do that!
Just one thing, could we also move the defaultCurrentScope & defaultIsolationScope there? I guess the same things apply there.

[timfish]

could we also move the defaultCurrentScope & defaultIsolationScope

Possibly. I was looking through hub.ts and it appears that much of that file will be gone for v8? Will defaultCurrentScope and defaultIsolationScope still be required?

[AbhiPrasad]

Will defaultCurrentScope and defaultIsolationScope still be required

it will as per getsentry/rfcs#122

[timfish]

Ah, I think it's the default* naming that confused me. These are simply the currentScope and isolationScope?

What happens when Hub is gone, will there need to be a new type to store the scopes that can be saved in the async storage? { current: Scope, isolation: Scope}

Ahhhhh ok, so are defaultCurrentScope and defaultIsolationScope only the current and isolation scopes when no async storage exists? ie. the fallback used in the browser (for now) and also when nothing was found in the async storage?

[mydea]

Ah, I think it's the default* naming that confused me. These are simply the currentScope and isolationScope?

What happens when Hub is gone, will there need to be a new type to store the scopes that can be saved in the async storage? { current: Scope, isolation: Scope}

Ahhhhh ok, so are defaultCurrentScope and defaultIsolationScope only the current and isolation scopes when no async storage exists? ie. the fallback used in the browser (for now) and also when nothing was found in the async storage?

They are basically the initial current/isolation scope - so if no async context is forked off yet, these will be the ones that are used - exactly! :)