Render <turbo-cable-stream-source> as display: none

Since the `<turbo-cable-stream-source>` is a Custom Element, it has a
default `style:` property of `display: inline`. As a visible element, it
participates in the page's layout.

This commit changes the element to always be `display: none` so that it
no longer participates in the layout.

app/assets/javascripts/turbo.js

@@ -4943,7 +4943,18 @@ function walk(obj) {
   }), {});
 }
 
+const template = Object.assign(document.createElement("template"), {
+  innerHTML: "<style>:host { display: none; }</style>"
+});
+
 class TurboCableStreamSourceElement extends HTMLElement {
+  constructor() {
+    super();
+    this.attachShadow({
+      mode: "open"
+    });
+    this.shadowRoot.appendChild(template.content.cloneNode(true));
+  }
   async connectedCallback() {
     connectStreamSource(this);
     this.subscription = await subscribeTo(this.channel, {