library/my_library/views/templates.xml

<?xml version="1.0" encoding="UTF-8"?>
<odoo>

    <template id="assets_end" inherit_id="web.assets_backend">
        <xpath expr="." position="inside">
            <script src="/my_library/static/src/js/field_widget.js" type="text/javascript" />
            <link href="/my_library/static/src/scss/field_widget.scss" rel="stylesheet" type="text/scss" />
        </xpath>
    </template>

</odoo>