Explicitly specify CSS export

[tobias-edwards]

Resolves #465

Explicitly specifies CSS export, inspired by vitejs/vite#2657 (comment)

However, vaul will always attempt to inject its CSS via <style>. A CSP policy that rejects 'unsafe-inline' will log an error here. Would be cleaner to conditionally turn this off or pass a nonce.

Usage (in Remix)

// app/root.tsx
import type { LinksFunction } from "@remix-run/node";
import vaulStyles from "vaul/style.css?url";

export const links: LinksFunction = () => [
  { rel: "stylesheet", href: vaulStyles },
];