[6.x] Forms 2: Provide pages array to {{ form:create }} tag

[duncanmcclean]

This pull request provides a pages array to the {{ form:create }} tag, allowing developers to loop through a form's pages on the frontend.

Forms always have at least one page internally. To add more pages, you'll need to install the first-party Forms Pro addon.

Usage

{{ form:contact }}
{{ pages }}
<h2>{{ display }}</h2>
{{ sections }}
<h3>{{ display }}</h3>
{{ form:fields }}
{{ field }}
{{ /form:fields }}
{{ /sections }}
<button>{{ button_label }}</button>
{{ ifnotfirst }}
<buttontype="button">{{ previous_page_label }}</button>
{{ /if }}
{{ /pages }}
{{ /form:contact }}

The following variables can be used when looping through pages:

Variable	Description
id	The page's handle.
display	The page's label.
instructions	The page's help text, if any.
button_label	Defaults to Next for every page but the last, and Submit for the last page.
previous_page_label	The "previous page" button label. Absent on the first page.
sections	The page's sections, with nested fields. The same variables are available as in the normal {{ sections }} loop.

JS Drivers can provide additional data (eg. an Alpine expression) by implementing addToRenderablePageData():

publicfunctionaddToRenderablePageData($page, $data)
{
return [
'show_page' => "...",
];
}

This mirrors the existing addToRenderableFieldData() hook. To avoid introducing breaking changes, this method is currently commented out in the JsDriver interface and can be un-commented in v7.

[duncanmcclean]

I've merged this into #14840