js: fix Javascript failures in Seamonkey/Palemoon.

Fix site Javascript failing to load in Seamonkey, Palemoon, and other
older browsers.

The @alpinejs/morph library uses public instance fields, which is ES2022
syntax not supported in older browsers. This is the code:

    var DomManager = class {
        el = void 0; // `el` is a public instance field
    }
    // => SyntaxError: bad method definition

The fix here is to separate the Alpine code into a separate bundle so
that a failure to load it doesn't cause the rest of the site's
Javascript to fail to load.

A better fix would be to either transpile the @alpinejs/morph library to
ES5 (which seems difficult to do in webpacker), or to fix the library
upstream to not use this syntax.

* https://inspiredwebdev.com/everything-new-in-es2022/
* https://blog.saeloun.com/2021/10/21/ecmacscript-public-instance-fields-and-private-instance-fields.html
* https://caniuse.com/?search=public%20class%20fields
* https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Public_class_fields#public_instance_fields

app/views/layouts/default.html.erb

@@ -23,9 +23,9 @@
 
   <%# XXX hack to only load Ruffle on Flash posts %>
   <% if controller_name == "posts" && action_name == "show" && @post&.is_flash? %>
-    <%= javascript_pack_tag "application", "flash", defer: false %>
+    <%= javascript_pack_tag "application", "alpine", "flash", defer: false %>
   <% else %>
-    <%= javascript_pack_tag "application", defer: false %>
+    <%= javascript_pack_tag "application", "alpine", defer: false %>
   <% end %>
 
   <%= stylesheet_pack_tag "application" %>