[VERY WIP] ref(tracing): Restructure Tracing Integration

[AbhiPrasad]

🚧 Please note this is a POC PR, and so is very messy

The actual PR will probably just be another branch with some cherry picked commits from this branch

This is a POC PR for the DACI I am working on to restructure the Tracing integration.

The PR is not based on master, yet is instead based on abhi/feat/tracing, a branch that added @sentry/tracing package. As such, all the changes in this PR are based this new @sentry/tracing package (in hindsight this was completely unnecessary as this is just a PR used for discussion, but I got a little ahead of myself :P)

The rough goals here:

• Refactor Tracing to be much more modular
• The concept of browser instrumentation and idle transactions should be separate (we can maybe then reuse idleTransaction in electron/react-native in the future)
• There should be an extensible TracingRouter class, that will create pageload/navigation transactions and handle everything related to that. Then any routing library can extend from this class and create their own pageload and navigation transactions
• WIP

Here are the rough changes here,

1. Create src/idleTransaction.ts
   • The IdleTransaction class extends the Transaction class, and just adds functionality to track activities
   • Every time a span is added, an activity is pushed
   • Everytime span.finish() is called, an activity is finished
   • When the number of activities goes to 0, the
   • autoPopAfter is removed from pushActivity, instead we put that into the span functionality (?)
   • WIP
2. Add idleTimeout?: number as argument for hub.startTransaction
   • if number is true, we create an IdleTransaction otherwise we create a Transaction
3. Create BrowserTracing integration in src/integrations/browsertracing
   • WIP
4. Create TracingRouter class that can be imported from src/integrations/router.ts
   • WIP
5. WIP