Feat: add support for one-page checkout

[ThbPS]

Questions	Answers
Description?	Adds support for switching between the classic 4-step checkout and one-page checkout. The checkout navigation is hidden when OPC is enabled, and a hidden toast hook is added for the account-created message after registration.
Type?	new feature
BC breaks?	no
Deprecations?	no
Fixed ticket?	N/A
Sponsor company	PrestaShop
How to test?	Enable one-page checkout, open the checkout page, and verify that the classic step navigation is not displayed. Create an account from checkout and verify that the account-created toast can be displayed after redirect. Then disable one-page checkout and verify that the classic 4-step checkout navigation still appears.

[ps-jarvis]

Hello @ThbPS!

This is your first pull request on hummingbird repository of the PrestaShop project.

Thank you, and welcome to this Open Source community!

[Hlavtox]

I think that there should be a separate template for OPC, this is a recipe for hell.

[ThbPS]

@Hlavtox

Since a module cannot cleanly override the theme’s checkout page template, achieving zero OPC dependency in checkout.tpl requires a Core change so that OrderController selects the page template directly.
Or are you suggesting a different approach?

[kpodemski]

@ThbPS

In my opinion, the OPC controller can incorporate its own templates/checkout/checkout.tpl and all the needed templates.

This means checkout.tpl would live directly in the OPC module, and you can have all control over the front-office part :)

Alternative option: I'm pretty sure you can extend this template from the module, and correctly use append or maybe even override certain blocks of this page, but, the cleanest way, as mentioned by @Hlavtox is to have a dedicated template, and, to follow my suggestion: directly inside the OPC module.