add ability to get user from initialize

[silesky]

This doesn't work:

      analytics.on('initialize', () =>  {
         user = analytics.user();
         console.log(user)
});

Currently, this works, but the semantics of .ready() means that it is not resilient to failed plugins. analytics.initialize should get the analytics instance as soon as its available, and all critical plugins are registered.

      analytics.ready(() => {
         console.log(analytics.user())
  });

If you just need access to the user in a way that is resillient to failed plugins, you can still do:

  analytics.register({
      type: 'utility',
      load(_ctx, analytics) {
             console.log(analytics.user()) 
       }
  })

[changeset-bot]

🦋 Changeset detected

Latest commit: f8e54f6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@segment/analytics-next	Minor

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.87%. Comparing base (6483541) to head (f8e54f6).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1286   +/-   ##
=======================================
  Coverage   82.87%   82.87%           
=======================================
  Files         225      225           
  Lines        6019     6019           
  Branches     1365     1365           
=======================================
  Hits         4988     4988           
  Misses       1031     1031           

Flag	Coverage Δ
browser	92.21% <100.00%> (ø)
core	89.86% <ø> (ø)
node	87.93% <ø> (ø)

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