Move common `@sentry/tracing` code to `@sentry/core`

[timfish]

Problem Statement

To work towards #5886 and #5815, common tracing code should be moved to @sentry/core.

Solution Brainstorm

Initially this should be done in a way that results in no breaking changes by re-exporting the types moved to core from tracing.

[timfish]

I'm working on a PR for this but there are a couple of minor issues still to resolve:

• Any types added to core conflict in nextjs/src/index.types.ts

sentry-javascript/packages/nextjs/src/index.types.ts

Lines 7 to 8 in 026072e

	export * from './server';
	export * from './edge';

• @sentry/tracing tests use private exports

To ensure there are no breaking changes, @sentry/tracing tests should stay with that package for now, however the tests use private exports like import { _addTracingExtensions } from '../src/hubextensions';

[AbhiPrasad]

@sentry/tracing tests use private exports

We can change the exports here?

[timfish]

We can change the exports here?

We could but they'll become part of the public API of @sentry/core which I guess is not desirable. Some of them are just constants used for testing.

We could import them directly from core:

import { _addTracingExtensions } from '../../../../core/src/tracing/hubextensions';