Check for .zshrc.local before appending to .zshrc

This will fix the issue where the main dotfiles repo keeps being modified
when the laptop script is executed. Using this method will allow for easier,
conflict free pulling of new dotfile updates as well.
I think `-w` is better, yeah. `-f` would ensure that it's not a directory, but I'm not that concerned about people somehow having a `~/.zshrc.local` directory.