Original PR description

The goal of this commit is to prepare ground to remove
lazytranslate function _lt() and keep only _t()
for a better understanding of the use of the translation function.

In this commit, 
- the translate function _t() has been updated to return the translation 
if they are loaded. If not, it throws an error.
- the lazytranslate function _lt() returns _t() function. 

**Corollaries :** 
Steps in test tours are now a function that returns an array of steps 
to avoid any interpolation of _t in this ones before translations has
been loaded. 

**Example :** 
```
registry.category("web_tour.tours").add("example", {
    test: true,
    steps: () => [
        {...},
        {...},
    ],
});
```
task-3292454

Community: https://github.com/odoo/odoo/pull/124157