Add package scripts

[purplecabbage]

Description

Templates might need to add their own npm-scripts to the project's package.json
This provides a helper function to write it for them.

Related Issue

Motivation and Context

ex.

addPkgScript(generator, {'pretest': 'echo ima run some tests now okey\?')

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

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

Checklist:

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

[codecov]

Codecov Report

Merging #61 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #61   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines          330       334    +4     
  Branches        41        42    +1     
=========================================
+ Hits           330       334    +4     

Impacted Files	Coverage Δ
lib/utils.js	100.00% <100.00%> (ø)

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