The format function was missing for the percentage widget type, so `field_utils.parse[this.formatType]` (see [1]) resolved as `undefined` which is not a function and so raised an error when the user was trying to update the field. This PR add that parsing function. Both `xx%` and `0.xx` are understood by the parser and resolve to the correct float value. opw-1895979 [1] https://github.com/odoo/odoo/blob/e220721a8ee80c02fe5aba5ed8c9f3f4ae54ea97/addons/web/static/src/js/fields/abstract_field.js#L399 closes odoo/odoo#28431