feat(tracing): Add tracing-internal package

[timfish]

This PR:

• Adds a tracing-internal package
• Moves all the code (minus side-effects) from @sentry/tracing to tracing-internal
• Moves tests that require private types to tracing-internal
• Ensures BrowserTracing is exported separately from tracing-internal for tree-shaking

[timfish]

The build passes fine on my machine. I've tried clearing the nx cache locally and get the same 🤔.

Instead of attempting to bundle the private @sentry-internal/tracing code into @sentry/tracing and getting all the typescript definitions bundled too... how about making @sentry-internal/tracing public and publishing it. It would be a lot less work for what is only a stop-gap solution.

[AbhiPrasad]

how about making @sentry-internal/tracing public and publishing it

let's just do this - we can leave a note in the readme that nobody should be installing this - and we don't have to publish it in the release registry.

[timfish]

Splitting into multiple PRs:

• feat(tracing): Add @sentry-internal/tracing package #7445
• feat(tracing): Move tracing code to internal package #7451