[FIX] web_widget_datepicker_options: Apply fixes after migration to v11

This applies some changes to the module after migration, to conform new
guidelines and conventions for version 11.0.

In addition, this applies the following fixes:
- The widged didn't work when the field was a `datetime` due to a typo
  in a JS conditional
- Replaced `Object.assign()` -> `_.assign()` to improve
  browser-compatibility

web_widget_datepicker_options/views/assets.xml

@@ -0,0 +1,12 @@
+<odoo>
+
+    <template id="assets_backend"
+        name="web_widget_datepicker_optionsassets"
+        inherit_id="web.assets_backend">
+        <xpath expr="." position="inside">
+            <script type="text/javascript"
+                src="/web_widget_datepicker_options/static/src/js/datepicker.js"></script>
+        </xpath>
+    </template>
+
+</odoo>