Use defualt zsh sitefndir

At the moment, zathura installs its zsh completions to "vendor-completions" in the zsh datadir. The zsh project uses "site-functions" for this purpose (sitefndir in their configure scripts), and even includes it at the top of user's fpath by default, allowing completions to work by default.

vendor-completions appears to be a nonstandard location used in some debian based distros. Could the default location for zsh completions be changed to site-functions please? As these distributions must already repackage site-functions completions into vendor-completions completions from nearly every other project, I think this is easier for all parties.