Prompt user when creating default project with env vars set

[childish-sambino]

Fixes

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Contributing to Twilio

All third party contributors acknowledge that any contributions they provide will be made under the same open source license that the open source project is provided under.

Short description of what this PR does:

• If the user has credential env vars set when adding/updating the default project, they are prompted to continue or cancel

If you have questions, please file a support ticket, or create a Github Issue in this repository.

[codecov]

Codecov Report

Merging #21 into master will increase coverage by 1.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   97.02%   98.04%   +1.01%     
==========================================
  Files          20       20              
  Lines         505      512       +7     
==========================================
+ Hits          490      502      +12     
+ Misses         15       10       -5

Impacted Files	Coverage Δ
src/commands/project/add.js	96.66% <100%> (+6.3%)	⬆️

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