Include GetVisualSelection() as Vim default

[Konfekt]

Dear @haya14busa ,

in https://stackoverflow.com/questions/1533565/how-to-get-visually-selected-text-in-vimscript/28398359#28398359 I propose your function as a robust mean to get the visual selection. There are many other, less robust functions, proposed.

It would be handy if there was such a robust function as yours to get the visual selection included in Vim by default; how about proposing it over at https://github.com/vim/vim/issues ? (If you do not think there are other, even better, say shorter and robuster, alternatives, perhaps in vital.vim.)