vencord: include package.json checked by Vesktop

Check for package.json is now being done in 1.5.6 of Vesktop, add this file to
the package output so the validity check passes.

See: d39c54b3ee
(cherry picked from commit 75f676844c)

pkgs/by-name/ve/vencord/package.nix

@@ -66,6 +66,7 @@ stdenv.mkDerivation (finalAttrs: {
   installPhase = ''
     runHook preInstall
 
+    cp package.json $out # Presence is checked by Vesktop.
     cp -r dist/${lib.optionalString buildWebExtension "chromium-unpacked/"} $out
 
     runHook postInstall