[12.x] Add config()->first() helper for fallback keys

[tabuna]

This PR adds a tiny, expressive first() method to the config repository (and thus the config() helper) that returns the first existing value from a list of keys, falling back to a default if none exist.

Motivation

Configuration keys sometimes move between files or get renamed in major releases / packages.
To maintain backward compatibility, packages and applications frequently do:

$value = config('new.key') ?? config('old.key') ?? 'default';

// or worse:
$value = collect(config()->get(['new.key', 'old.key'], 'default'))->first();

The second variant is especially noisy in Blade:

@extends(collect(config()->get(['layout', 'theme.layout'], 'app'))->first())

Proposed solution

Add first() method:

$value = config()->first(['new.key', 'old.key'], 'default');

@extends(config()->first(['layout', 'theme.layout'], 'app'))