stdenv: support mainProgram in meta

Support `mainProgram` as an attribute of `meta` for packages.

This is an attribute used by [`nix
run`](https://nixos.org/manual/nix/unstable/command-ref/new-cli/nix3-run.html#description)
to customize the main program of a package.

For example, `pkgs.neovim` provides a `/bin/nvim` executable which users
would (almost certainly) prefer `nix run` to execute instead of a
non-existing `/bin/neovim`.

Signed-off-by: Ana Hobden <operator@hoverbear.org>

pkgs/stdenv/generic/check-meta.nix

@@ -203,6 +203,7 @@ let
   metaTypes = with lib.types; rec {
     # These keys are documented
     description = str;
+    mainProgram = str;
     longDescription = str;
     branch = str;
     homepage = either (listOf str) str;