feat(tracing): Move common tracing code to core

[timfish]

Closes #7091

This PR:

• Moves the common tracing code to core/src/tracing
  • @sentry/core now exports the same common types plus:
    • addTracingExtensions - Required so that @sentry/tracing can still add the extensions via addExtensionMethods
    • TRACING_DEFAULTS - This contains the timing defaults that are used across tracing
• Leaves addExtensionMethods in @sentry/tracing due to _autoloadDatabaseIntegrations
• Leaves the tests in @sentry/tracing to ensure there are no breaking changes to public API
  • idletransaction.test.ts imports code from core so IdleTransactionSpanRecorder doesn't need to be made public
• I had to make some changes to the tests where Jest mocks were getting hoisted

This PR does not:

• Change any of the downstream SDKs to import Span, Transaction, etc from @sentry/core rather than @sentry/tracing

[timfish]

Well at least the integration tests are doing their job...

[AbhiPrasad]

Gonna take this over to try and merge!

[AbhiPrasad]

Closing this PR in favour of #7339 (same thing, but with merge conflicts fixed. Had to open a new PR since I couldn't push to fork).