Chat Federation: using S2S if available.

* if both local and remote instance have external XMPP connections enabled, the user joins the remote room with his local account
* some code refactoring (builtin.ts)

Note: documentation and settings descriptions are to do.

Related to #112

conversejs/lib/dom.ts

@@ -0,0 +1,25 @@
+import type { InitConverseParams } from './types'
+
+function initDom ({ forceReadonly, transparent }: InitConverseParams, isInIframe: boolean): void {
+  const body = document.querySelector('body')
+  if (isInIframe) {
+    if (body) {
+      body.classList.add('livechat-iframe')
+      // prevent horizontal scrollbar when in iframe. (don't know why, but does not work if done by CSS)
+      body.style.overflowX = 'hidden'
+    }
+  }
+  if (forceReadonly) {
+    body?.classList.add('livechat-readonly')
+    if (forceReadonly === 'noscroll') {
+      body?.classList.add('livechat-noscroll')
+    }
+  }
+  if (transparent) {
+    body?.classList.add('livechat-transparent')
+  }
+}
+
+export {
+  initDom
+}