Restore "unread" tab Attribute

This restore the `unread` tab Attribute that was removed all the way back in Firefox 61.
This is useful for theming since it allows custom CSS styling for tabs that has been opened in the background but haven't been actually viewed yet.

https://bugzilla.mozilla.org/show_bug.cgi?id=1453957

browser/components/tabbrowser/content/tabbrowser.js

@@ -1312,6 +1312,7 @@
       if (!this._previewMode) {
         newTab.recordTimeFromUnloadToReload();
         newTab.updateLastAccessed();
+        newTab.removeAttribute("unread");
         oldTab.updateLastAccessed();
         // if this is the foreground window, update the last-seen timestamps.
         if (this.ownerGlobal == BrowserWindowTracker.getTopWindow()) {
@@ -6997,6 +6998,9 @@
 
             this.mTab.setAttribute("bursting", "true");
           }
+
+          if (!this.mTab.selected)
+            this.mTab.setAttribute("unread", "true");
         }
 
         if (this.mTab.hasAttribute("progress")) {