Customize styling of the porting embed

[timomeh]

• Adds the ability to pass in custom class names based on a class name function
  • This supports all kinds of custom stylings: CSS Modules, css-in-js, Tailwind, plain css
• Still ships with some unique class names (GigsEmbeds-XYZ) so we can ship a default CSS file
• Adds an option to use a custom formId, which is used together with the submit button
• Adds an stepChange event which is fired when the wizard step changes

{
  formId: 'myCustomFormId',
  className: {
    form: ({ name, touched, dirty, valid, submitting }) => 'custom-class-name',

    // the actual input
    input: ({ name, touched, dirty, valid }) => 'custom-class-name',

    // the input label
    label: ({ name, touched, dirty, valid }) => 'custom-class-name',

    // the field error
    error: ({ name, touched, dirty }) => 'custom-class-name',

    // the field container (div around input, label, error)
    field: ({ name, touched, dirty, valid }) => 'custom-class-name',
  }
}