Add a shell script to replace the PrepareMetalHeaders plugin

[louen]

This PR is a fix for issue #156.
It replaces the swift package build plugin PrepareMetalShaders with a shell script run by the package maintainer.

The shell script can be run as standalone; as set up it is called from update-mlx.sh

• ran update-mlx.sh and `pre-commit
• checked the new package builds with Xcode (compiled the libraries and rand example app)
• checked the newly generated folder is exactly the same than the one coming out of PrepareMetalShaders
• new shell script passes shellcheck without errors or warnings

[davidkoski]

Awesome, thank you!!