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.
So none of this code executes if `~/.bin` is already in `$PATH`. That means that what happens if the line's already in the file, but doesn't match the current environment variable is already an edge-case not worth worrying about much IMO.

Even in the case that it's already in the file, I'd assume the user put it there, in an appropriate, non-clobbered place, and laptop shouldn't add a duplicate line to the end.

So I think the semantics of `append_to_zshrc` are appropriate here too.