Add tab, view to AppHomeOpenedEvent

[seratch]

Summary

Although the API documentation team is still working on the updates on the page (the updates will come very soon), app_home_opened event has two more fields - tab and view as announced at the Spec conference yesterday (Refer to https://api.slack.com/surfaces/tabs for details of App Home).

This pull request adds those to the type definition to better support it in TypeScript.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #292 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #292   +/-   ##
=======================================
  Coverage   73.71%   73.71%           
=======================================
  Files           7        7           
  Lines         506      506           
  Branches      145      145           
=======================================
  Hits          373      373           
  Misses        101      101           
  Partials       32       32

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