testing/friendly-snippets: new aport

Although friendly-snippets are used mainly by vim/nvim users, the package itself is not specific to vim/nvim. Snippet plugins for other editors can use them as well. The snippets are installed in /usr/share/friendly-snippets, and users can configure their plugins to load from aforementioned directory.

I also added a note in post-install script about how to configure it with luasnip, as an example.