Add .Defines to js.Build options

This is needed to import `react` as a library, e.g.:

```
{{ $jsx := resources.Get "index.jsx" }}
{{ $options := dict "defines" (dict "process.env.NODE_ENV" "\"development\"") }}
{{ $js := $jsx | js.Build $options }}
```

Fixes #7489

resources/resource_transformers/js/build_test.go

@@ -42,6 +42,7 @@ func TestToInternalOptions(t *testing.T) {
 		JSXFactory:  "v3",
 		JSXFragment: "v4",
 		Externals:   []string{"react"},
+		Defines:     map[string]interface{}{"process.env.NODE_ENV": "production"},
 		Minify:      true,
 	}
 
@@ -52,7 +53,7 @@ func TestToInternalOptions(t *testing.T) {
 		JSXFactory:  "v3",
 		JSXFragment: "v4",
 		Externals:   []string{"react"},
-		Defines:     nil,
+		Defines:     map[string]string{"process.env.NODE_ENV": "production"},
 		TSConfig:    "",
 	})