feat: add artalk comment module (#5868)

server/modules/comments/artalk/code.yml

+main: |
+  <div id="artalk-container"></div>
+head: |
+  <link href="{{server}}/dist/Artalk.css" rel="stylesheet">
+  <script src="{{server}}/dist/Artalk.js"></script>
+body: |
+  <script>
+    window.onload = function() {
+      new Artalk({
+        el:        '#artalk-container',
+        pageKey:   '{{pageId}}',
+        pageTitle: '',
+        server:    '{{server}}',
+        site:      '{{siteName}}',
+      });
+    };
+  </script>

server/modules/comments/artalk/definition.yml

+key: artalk
+title: Artalk
+description: A light-weight self-hosted comment system.
+author: CDN18
+logo: https://static.requarks.io/logo/artalk.png
+website: https://artalk.js.org
+codeTemplate: true
+isAvailable: true
+props:
+  server:
+    type: String
+    title: Artalk Backend URL
+    default: ''
+    hint: 'Publicly accessible URL of your Artalk instance. It should start with http/https and omit the trailing slash. (e.g. https://artalk.example.com)'
+    maxWidth: 650
+    order: 1
+  siteName:
+    type: String
+    title: Site Name
+    default: ''
+    hint: 'The name of this site configured in the artalk backend. Leave empty to use default site.'
+    maxWidth: 450
+    order: 2