Rewrite node_modules copying in Bash

[kdmccormick]

Background

Part of:

• Build edx-platform assets without Paver #31604

Subtasks

• Create a sh (not bash) script scripts/assets/copy-node-modules.sh, re-implementing the functionality of pavelib/assets.py:process_npm_assets.
• Replace the implemention of pavelib/assets.py:process_npm_assets with a simple call to the new shell script.

Notes

...

[kdmccormick]

Would be resolved by #31519

[kdmccormick]

Closed in favor of the parent issue: #31604