[11.x] Fix prompt fallback return value when using numeric keys

[jessarcher]

This PR fixes an inconsistency issue with any Prompt fallbacks that rely on Symfony's choice component.

The inconsistency occurs when passing an array of options with numeric keys that are not sequential, starting from 0.

Prompts uses PHP's array_is_list function to determine whether the returned value should be the selected key or the selected value. Symfony's choice component differs by always returning the value for any array with all numeric keys, regardless of whether they are sequential from 0:

// Prompts without fallback

select('Foo', [
    '9999' => 'High', // returns '9999'
    '1' => 'Low', // returns '1'
]);

// Prompts with Symfony fallback

select('Foo', [
    '9999' => 'High', // returns 'High'
    '1' => 'Low', // returns 'Low',
]);

select('Foo', [
    '9999' => 'High', // returns '9999'
    'medium' => 'Medium', // returns 'medium',
    '1' => 'Low', // returns '1'
]);

This is an issue because the return value is inconsistent depending on whether or not the fallback was activated.

This PR solves this by wrapping each option in a new PromptOption class and passing them to Symfony as a list. Symfony can render the string version of the PromptOption class, and because it's passed as a list, it will always return the selected PromptOption class, allowing us to extract the correct value.

A nice side effect of always passing a list is that Symfony will not display the underlying array keys to the user, which makes the fallback more consistent with Prompts. It also simplifies the approach we use to inject the "None" option for the multiselect fallback.

[jessarcher]

I made this method private because it ends up in the Command class, and I wanted to prevent IDEs from suggesting it.

[driesvints]

@jessarcher we fixed the tests on the base branch, can you rebase?