feat(tvix/eval): allow extending builtins outside of tvix_eval

The change allows applications that use tvix_serde for parsing
nix-based configuration to extend the language with domain-specific
set of features.

Change-Id: Ia86612308a167c456ecf03e93fe0fbae55b876a6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8848
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/Cargo.nix

@@ -8169,6 +8169,13 @@ rec {
             packageId = "tvix-eval";
           }
         ];
+        devDependencies = [
+          {
+            name = "genawaiter";
+            packageId = "genawaiter";
+            usesDefaultFeatures = false;
+          }
+        ];
 
       };
       "tvix-store" = rec {