feat(mixpanel): add support of simplified api

[Gauravudia]

Description of the change

Add support for Simplified ID merge API

• Drop $merge event support when Simplified ID merge is selected
• Remove support of alias call when Simplified ID merge is selected
• Add $device_id and $user_id when Simplified ID merge is selected

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Related issues

Fix #1

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers mentioned in a comment
• Changes have been reviewed by at least one other engineer
• Issue from task tracker has a link to this pull request

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b3e5d82) 87.48% compared to head (87a03fe) 87.49%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2042   +/-   ##
========================================
  Coverage    87.48%   87.49%           
========================================
  Files          501      501           
  Lines        25226    25237   +11     
  Branches      6322     6331    +9     
========================================
+ Hits         22069    22080   +11     
  Misses        2819     2819           
  Partials       338      338           

Impacted Files	Coverage Δ
src/v0/destinations/mp/transform.js	98.37% <100.00%> (+0.08%)	⬆️
src/v0/destinations/mp/util.js	100.00% <100.00%> (ø)

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