[FEAT] Pre-update user script

[NeonDaniel]

Objective

Provide a method for users to specify an arbitrary script to run before performing a SquashFS update (similar to the post-update script).

Initial Implementation Requirements

• Implement a script call before restarting the device

Other Considerations

• Decide if the pre-update script can run while Neon is running or there is reason to run after Neon services have stopped.
• Decide if the script runs immediately upon squashFS update download or in a separate handler that the update skill will call.
• As discussed in Matrix chat