neovim-qt: Add meta.mainProgram

Add a `meta.mainProgram` attribute to allow use of `lib.getExe` properly
with this program.

pkgs/applications/editors/neovim/neovim-qt.nix

@@ -37,6 +37,7 @@ mkDerivation rec {
     description = "Neovim client library and GUI, in Qt5";
     homepage = "https://github.com/equalsraf/neovim-qt";
     license     = licenses.isc;
+    mainProgram = "nvim-qt";
     maintainers = with maintainers; [ peterhoeg ];
     inherit (neovim.meta) platforms;
   };