Use hardcoded connection string fallback rather than .env file (#348)

Seems like npm filters out `.env` files when publishing. It's probably a
good thing on the whole, but we actually want them included for
`create-y-sweet-app` because we copy them over when bootstrapping a new
project.

Rather than try to work around the issue, I just removed the `.env`
files and set up the document managers like this:

```js
const manager = new DocumentManager(
  process.env.CONNECTION_STRING || "ys://127.0.0.1:8080",
);
```

As a plus, it's a little more explicit what's happening in the code.

js-pkg/create-y-sweet-app/src/frameworks/nextjs/app/page.tsx

@@ -3,7 +3,9 @@ import { YDocProvider } from "@y-sweet/react";
 
 import { App } from "@/components/App";
 
-const manager = new DocumentManager(process.env.CONNECTION_STRING!);
+const manager = new DocumentManager(
+  process.env.CONNECTION_STRING || "ys://127.0.0.1:8080",
+);
 
 export default async function Home() {
   const docId = "my-doc-id";

js-pkg/create-y-sweet-app/src/frameworks/remix/app/lib/ysweet.server.ts

@@ -1,3 +1,5 @@
 import { DocumentManager } from "@y-sweet/sdk";
 
-export const manager = new DocumentManager(process.env.CONNECTION_STRING!);
+export const manager = new DocumentManager(
+  process.env.CONNECTION_STRING || "ys://127.0.0.1:8080",
+);