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.
Thoughts on reducing even more duplication?

```sh
append_to_zshrc() {
  local text="$1" zshrc

  # set the right $zshrc

  if ! grep -Fqs "$text" "$zshrc"; then
    printf "%s\n" "$text" >> "$zshrc"
  fi
}

append_to_zshrc 'export PATH="..."'
append_to_zshrc 'eval $(rbenv... )'
```

?