zshrc_path() function to return .zshrc.local path

This function will return the path of .zshrc.local if it exists.
It will return .zshrc if there is no .local dotfile.
Alternatively, you could just add the return value and use it like:

```sh
if append_to_zsh '# recommended by brew doctor'; then
  append_to_zsh 'export ...'
  export PATH="..."
fi
```