Move tracing code to core without impacting bundle size

[timfish]

This was created on top of the hub-to-core migration PR (getsentry#5823) which further moves all the runtime agnostic tracing code to core too.

All I'm really doing here is moving the source files over and fixing up all the imports/exports.

• Since none of the core code references any of the moved tracing code, bundling results in exactly the same browser bundle as before
• This builds on top of another unmerged PR, the full changelog from master is huge
• The tests don't pass because I haven't made an attempt to move them
• All that's left in tracing is the node/browser specific code