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.
Hmm. That is tricky.

We could change the append function slightly to make this a bit easier:

- Let it take multiple lines and do the check-append in a loop
- Return successfully if (all?, any? I dunno) lines were actually appended, unsuccessfully if not

Then you could do things like

```sh
if append_to_zsh '...' '...'; then
  fancy_echo "Added Homebrew location earlier in PATH ..."
  export PATH="..."
fi
```

If that's worth it or not, I'm not sure. Depends how crazy `append_to` gets to have this behavior.