Assign window.Turbo in @hotwired/turbo-rails/index

With the changes made in the prior commit to pass CI merged, the fact
that Turbo and Turbo Rails somehow have differing references to `Turbo`
and `window.Turbo`.

This commit reverts the test harness change that sets `window.Turbo`,
and instead assigns that value as part of the
`@hotwired/turbo-rails/index` module:

```diff
-import { Turbo } from "@hotwired/turbo-rails"
-
-window.Turbo = Turbo
+import "@hotwired/turbo-rails"
```

app/assets/javascripts/turbo.js

@@ -4810,7 +4810,7 @@ window.Turbo = Turbo;
 
 start();
 
-var turbo_es2017Esm = Object.freeze({
+var Turbo$1 = Object.freeze({
   __proto__: null,
   FetchEnctype: FetchEnctype,
   FetchMethod: FetchMethod,
@@ -4981,6 +4981,8 @@ function isBodyInit(body) {
 
 addEventListener("turbo:before-fetch-request", encodeMethodIntoRequestBody);
 
+window.Turbo = Turbo$1;
+
 var adapters = {
   logger: self.console,
   WebSocket: self.WebSocket
@@ -5514,4 +5516,4 @@ var index = Object.freeze({
   getConfig: getConfig
 });
 
-export { turbo_es2017Esm as Turbo, cable };
+export { Turbo$1 as Turbo, cable };

app/javascript/turbo/index.js

@@ -9,3 +9,5 @@ export { cable }
 import { encodeMethodIntoRequestBody } from "./fetch_requests"
 
 addEventListener("turbo:before-fetch-request", encodeMethodIntoRequestBody)
+
+window.Turbo = Turbo

test/dummy/app/javascript/application.js

@@ -1,4 +1,2 @@
 // Configure your import map in config/importmap.rb. Read more: https://github.com/rails/importmap-rails
-import { Turbo } from "@hotwired/turbo-rails"
-
-window.Turbo = Turbo
+import "@hotwired/turbo-rails"