feat(SFT-1311): added a form variable to the field rendering variables

[seandelaney]

Form info is now available within a field:

{exp:freeform_next:form form="myForm"}
    Fields Info
    <ul>
        {rows}
            {fields}
                <li>
                    Field
                    <ul>
                        <li>Label: {field:label}</li>
                        <li>Handle: {field:handle}</li>
                        <li>Type: {field:type}</li>
                        <li>
                            Form:
                            {field:form}
                            <ul>
                                <li>ID: {form:id}</li>
                                <li>Name: {form:name}</li>
                                <li>Handle: {form:handle}</li>
                                <li>Description: {form:description}</li>
                                <li>Return URL: {form:return_url}</li>
                                <li>Return: {form:return}</li>
                                <li>Action: {form:action}</li>
                                <li>Method: {form:method}</li>
                                <li>Class: {form:class}</li>
                                <li>Page Count: {form:page_count}</li>
                            </ul>
                            {/field:form}
                        </li>
                    </ul>
                </li>
            {/fields}
        {/rows}
    </ul>
{/exp:freeform_next:form}