Fixed broken building task of combineModules

[sakshamsaxena]

Resolves #3883

Changes:

• Updated the logic to generate custom build by creating a clone of app.js temporarily but with only the required modules mentioned as imports. This had to be done so as to introduce a single file entry point for bundling, as otherwise the p5 class was not exported as it should, and its bindings were lost.

PR Checklist

• npm run lint passes
• Inline documentation is included / updated
• Unit tests are included / updated
• Benchmarks are included / updated

[lmccart]

thanks @sakshamsaxena!