mass_editing_module_for_odoo/mass_editing/views/template.xml

<odoo>
    <data>
        <template id="sale_order_backend" name="sale_order assets" inherit_id="web.assets_backend">
            <xpath expr="." position="inside">
               <script type="text/javascript" src="/mass_editing/static/src/js/mass_editing.js"></script>
            </xpath>
        </template>
    </data>
</odoo>