Added form validation models

Themes can utilize the simple form validation models as

let fieldValue = $('input[name="username"]'); //example value is "myname"

var validated = utils.forms.login.username(fieldValue)
console.log(validated) //false, Bigcommerce requires email based usernames, utils.forms.login.username performs these checks for you.

Available form models will be documented and include forms such as Login, Create Account, Add Address, etc.