Rails 8: Include manifest.js in install generator

[mamhoff]

We found we need to do this for dummy apps, but we also need to do it for generated apps.

Checklist

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• I agree that my PR will be published under the same license as Solidus.
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• 📖 I have updated the README to account for my changes.
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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.67%. Comparing base (37a5c8a) to head (12a6033).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...ib/generators/solidus/install/install_generator.rb	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6122      +/-   ##
==========================================
- Coverage   88.68%   88.67%   -0.01%     
==========================================
  Files         830      830              
  Lines       18013    18015       +2     
==========================================
  Hits        15975    15975              
- Misses       2038     2040       +2     

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[kennyadsl]

Thanks, Martin!