fix(core): make app.js optional as initially intended

[faustbrian]

Summary

Adds the app.js file to testnet and fixes the wrongly behaviour that the app.js file is required to be published. It was intended to be optional and fallback to the default app.js that core ships if it doesn't exist in the published configuration folder.

Can be tested with yarn global add @arkecosystem/core@2.6.0-alpha.1.

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (2.6@bf16abd). Click here to learn what that means.
The diff coverage is 0%.

@@          Coverage Diff           @@
##             2.6    #3510   +/-   ##
======================================
  Coverage       ?   66.12%           
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  Files          ?      439           
  Lines          ?    12468           
  Branches       ?     1712           
======================================
  Hits           ?     8245           
  Misses         ?     4189           
  Partials       ?       34

Impacted Files	Coverage Δ
packages/core/src/utils.ts	0% <0%> (ø)

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