Include basic theme's JS files

I wanted to optimize the number of downloads we make, but ReadTheDocs
assumes that all documentation pages include jQuery.

src/furo/theme/base.html

@@ -64,9 +64,15 @@
 
     {# Custom JS #}
     {%- block extrajs -%}
+    {# This is *exactly* how `basic` handles this file. #}
+    <script id="documentation_options" data-url_root="{{ pathto('', 1) }}" src="{{ pathto('_static/documentation_options.js', 1) }}"></script>
     {%- for path in script_files -%}
-      {# We don't want Sphinx's hard-coded scripts #}
-      {%- if path not in ["_static/jquery.js", "_static/underscore.js", "_static/doctools.js", "_static/language_data.js"] -%}
+      {# We want Sphinx's hard-coded scripts to be loaded synchronously #}
+      {%- if path in ["_static/jquery.js", "_static/underscore.js"] -%}
+        <script src="{{ pathto(path, 1) }}" async></script>
+      {% elif path in ["_static/doctools.js", "_static/language_data.js"] %}
+        <script src="{{ pathto(path, 1) }}" defer></script>
+      {% else %}
         <script src="{{ pathto(path, 1) }}"></script>
       {%- endif -%}
     {% endfor -%}

src/furo/theme/search.html

@@ -1,12 +1,6 @@
 {% extends "base.html" %}
 
 {%- block extrajs -%}
-<script id="documentation_options" data-url_root="{{ pathto('', 1) }}" src="{{ pathto('_static/documentation_options.js', 1) }}"></script>
-{%- for path in script_files -%}
-  {# We want Sphinx's hard-coded scripts, since they're needed for
-     search to work. #}
-  <script src="{{ pathto(path, 1) }}"></script>
-{% endfor -%}
 <script src="{{ pathto('_static/searchtools.js', 1) }}"></script>
 {%- endblock extrajs-%}