Fix importing as ESM by using conditional exports

This fixes the following issue when exporting the site as a static
bundle using SvelteKit.

```
> Using @sveltejs/adapter-static
> Named export 'SvelteToast' not found. The requested module '@zerodevx/svelte-toast' is a CommonJS module, which may not support all module.exports as named exports.
CommonJS modules can always be imported via the default export, for example using:

import pkg from '@zerodevx/svelte-toast';
const {SvelteToast} = pkg;
```

Author: kjagiello

package.json

@@ -6,6 +6,10 @@
   "svelte": "src/index.js",
   "module": "dist/index.mjs",
   "main": "dist/index.js",
+  "exports": {
+    "require": "./dist/index.js",
+    "import": "./dist/index.mjs"
+  },
   "scripts": {
     "tailwind:watch": "chokidar 'docs/tailwind*.*' -c 'npm run tailwind:build'",
     "tailwind:build": "tailwind build docs/tailwind.css -c docs/tailwind.config.js -o docs/build/global.css",

src/index.js

@@ -1,4 +1,2 @@
-import SvelteToast from './SvelteToast.svelte'
-import { toast } from './stores'
-
-export { SvelteToast, toast }
+export { default as SvelteToast } from './SvelteToast.svelte'
+export { toast } from './stores'