undeploy hook for events, with tests

[purplecabbage]

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[codecov[bot]]

Codecov Report

Merging #678 (489412d) into master (bf98c2f) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #678   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           57        57           
  Lines         3128      3133    +5     
  Branches       585       587    +2     
=========================================
+ Hits          3128      3133    +5     

Impacted Files	Coverage Δ
src/commands/app/undeploy.js	100.00% <100.00%> (ø)

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