chore: move playground to eslint-online-playground (sveltejs#829)

This PR is part of
sveltejs#824. It
represents the first step in the omnisite.

In this update, I switched playground to
[eslint-online-playground](https://eslint-online-playground.netlify.app/#eslint-plugin-svelte%20with%20typescript).
The transition to `twoslash-eslint` will be made in the next PR. After
tht I think we can remove all ESLint runners from the documentation site
to simplify the build process.

Author: baseballyama

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -79,7 +79,7 @@ body:
       placeholder: |
         https://github.com/[your]/[repo]
         or
-        https://sveltejs.github.io/eslint-plugin-svelte/playground/#[hash]
+        https://eslint-online-playground.netlify.app/#[hash]
     validations:
       required: true
   - type: textarea

README.md

@@ -2,7 +2,7 @@
 
 `eslint-plugin-svelte` is the official [ESLint] plugin for [Svelte].  
 It provides many unique check rules by using the template AST.  
-You can check on the [Online DEMO](https://sveltejs.github.io/eslint-plugin-svelte/playground/).
+You can check on the [Online DEMO](https://eslint-online-playground.netlify.app/#eslint-plugin-svelte%20with%20typescript).
 
 **_We are working on experimental support for Svelte v5, but may break with new versions of Svelte v5._**