fix: Add svelte exports condition (#85)

Fixes the following warning:

```
[vite-plugin-svelte] WARNING: The following packages have a svelte field in their package.json but no exports condition for svelte.

svelte-markdown@0.4.0

Please see https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/faq.md#missing-exports-condition for details.
```

package.json

@@ -6,6 +6,13 @@
  "module": "dist/sveltemarkdown.es.js",
  "jsnext:main": "dist/sveltemarkdown.es.js",
  "svelte": "src/index.js",
+ "exports": {
+ ".": {
+ "types": "./types/index.d.ts",
+ "svelte": "./src/index.js"
+ },
+ "./package.json": "./package.json"
+ },
  "type": "module",
  "files": [
  "dist",