feat: add bundled app validation to ensure that package.json and main entry point exist

[malept]

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• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

More than once in the Discord server I've had to help someone debug their app bundling, and it turned out that their ignore settings ignored either their package.json file or their main entry point script (e.g., main.js). This PR adds a validation check before it generates an ASAR to make sure that both exist.

[codecov]

Codecov Report

Merging #1257 (c447a69) into master (a82345b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1257   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          698       715   +17     
=========================================
+ Hits           698       715   +17     

Impacted Files	Coverage Δ
src/common.js	100.00% <100.00%> (ø)
src/platform.js	100.00% <100.00%> (ø)

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