Add a check to make sure that `~/Library/LaunchAgents` acually exists

[sethgoldin]

When a new user is created on a fresh macOS system, ~/Library/LaunchAgents isn't automatically created.

This script should be amended with a mkdir -p ~/Library/LaunchAgents line to make sure to create the folder if it doesn't already exist.

[sethgoldin]

This was fixed with this commit: 21f21ff