[11.x] Add Fluent::set method

[stevebauman]

Description

This PR adds a chainable set method to the Support\Fluent class.

This allows a developer to use dot notation to set deeply nested attributes on the Fluent instance.

Usage

$fluent = new Fluent;

// Set basic attributes
$fluent->set('product', 'iPhone')
       ->set('version', 15)
       ->set('developer', 'Apple');

// Use dot notation for nested attributes
$fluent->set('specs.color', 'Space Black')
       ->set('specs.storage', '256GB')
       ->set('specs.price.usd', 1199);

// Retrieve values
echo $fluent->product; // "iPhone"
echo $fluent->specs['color']; // "Space Black"
echo $fluent->specs['price']['usd']; // 1199

// Retrieve values using get with dot notation
echo $fluent->get('specs.color'); // "Space Black"
echo $fluent->get('specs.price.usd'); // 1199