added linux_{default,latest} to linuxKernel.kernels

this improves discoverability, as many users (including me) have been confused why only custom kernels are available

fixes #260636

pkgs/top-level/linux-kernels.nix

@@ -221,6 +221,10 @@ in {
        then latest
        else testing;
 
+    linux_default = packageAliases.linux_default.kernel;
+
+    linux_latest = packageAliases.linux_latest.kernel;
+
     # Using zenKernels like this due lqx&zen came from one source, but may have different base kernel version
     # https://github.com/NixOS/nixpkgs/pull/161773#discussion_r820134708
     zenKernels = callPackage ../os-specific/linux/kernel/zen-kernels.nix;