What needs to be done?

In order to complete #1912 we first need to add type_html to flow-php/types library that would follow the convention of other types allowing for assertions, casting and validation.

type_html()->toString() should simply return html

All requirements about types are documented here:

• https://flow-php.com/documentation/components/libs/types/
• https://flow-php.com/documentation/components/libs/types/architecture/