Add support for handling CORS preflight requests

[jurakin]

Description

This pull request adds support for handling CORS preflight requests in the routing system. The router will respond to OPTIONS requests only when the route actually exists and a preflight setting is enabled. The setting is disabled by default.

Changes

• Added a preflight setting to enable handling of preflight requests.
• When the setting is enabled, the router responds to OPTIONS requests with status 200 OK and no content, following the MDN documentation.
• Added unit tests and README documentation.

Usage

// single route
SimpleRouter::form('foo', function () {
    // ...
})->setSettings(['preflight' => true]);

// group routes
SimpleRouter::group(['preflight' => true], function () {
    SimpleRouter::form('foo', function() {
        // ...
    });
});

• Requesting OPTIONS /foo will return HTTP/1.1 200 OK.
• Requesting POST /foo will proceed normally.