Initial commits for module validation

[BitcoinMitchell]

Description

This PR prepares the module to be validated by the PrestaShop team.

These changes are based on the Prestashop Validation Checklist.

Type of change

• Refactor (non-breaking change which improves the codebase)

How Has This Been Tested?

Module was build and tested on a local Prestashop store.

Test Configuration

• BTCPay Server version: 1.12.5
• PrestaShop version: 8.1.3
• PHP version: 8.1.27

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Test zip: v6.1.2

[ndeet]

Only read the code but did not test locally. Those constant checks are common but those index.php in the subfolders with header('Location: ../') was new to me, normally it is just an empty index.php but for sure there are "reasons" :D