Try to make the error nicer when mixing ESM and CommonJS

We define `module` and `exports` on the global object, so that *if* the
executed code isn't part of CommonJS which defines them, it will access
the global ones.

The global ones just error out on access with a message pointing out
that this likely is due to mixing CommonJS and ESM.

js/bundle.go

@@ -436,6 +436,20 @@ func (b *Bundle) setInitGlobals(rt *sobek.Runtime, vu *moduleVUImpl, modSys *mod
 		}
 		return openImpl(rt, b.filesystems["file"], pwd, filename, args...)
 	})
+	warnAboutModuleMixing := func(name string) {
+		warnFunc := rt.ToValue(func() error {
+			return fmt.Errorf(
+				"you are trying to access identifier %q, this likely is due to mixing ESM and CommonJS syntax."+
+					"This isn't supported in the JavaScript standard, please use only one or the other",
+				name)
+		})
+		err := rt.GlobalObject().DefineAccessorProperty(name, warnFunc, warnFunc, sobek.FLAG_FALSE, sobek.FLAG_FALSE)
+		if err != nil {
+			panic(fmt.Errorf("failed to set '%s' global object: %w", name, err))
+		}
+	}
+	warnAboutModuleMixing("module")
+	warnAboutModuleMixing("exports")
 }
 
 func generateFileLoad(b *Bundle) modules.FileLoader {

js/module_loading_test.go

@@ -168,7 +168,7 @@ func TestLoadExportsIsntUsableInModule(t *testing.T) {
 			initVU, err := r.NewVU(ctx, 1, 1, ch)
 			require.NoError(t, err)
 			vu := initVU.Activate(&lib.VUActivationParams{RunContext: ctx})
-			require.ErrorContains(t, vu.RunOnce(), "ReferenceError: exports is not defined")
+			require.ErrorContains(t, vu.RunOnce(), `you are trying to access identifier "exports"`)
 		})
 	}
 }