[4.4] refactor: a single point of sending the Response.

[iRedds]

Description
This PR refactors the CodeIgniter class code so that all successful responses, except those caused by exceptions, are sent from one place.

• The CodeIgniter::handleRequest() method now always returns a Response.
• Moved HTTP cli check and page cache check to CodeIgniter::handleRequest() method. Because these methods generate a Response and are actually part of the request processing.
• Event call Events::trigger('pre_system') moved to method CodeIgniter::handleRequest() and is now on the same "level" as Events::trigger('post_system')
• The CodeIgniter::$returnResponse property has been deprecated as it is no longer used.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[iRedds]

I'm a bit surprised that psalm can't find the Tests\Support\Controllers\Popcorn class.
Moreover, he has no claims to the same class from the neighboring test.

[kenjis]

I don't know why but rarely Psalm reports errors that cannot reproduce.
I cannot reproduce the following error:

Run vendor/bin/psalm
Target PHP version: 7.4 (inferred from composer.json).
Scanning files...
Analyzing files...

░░░░░░░░░░░░░░░░░░░░░░

ERROR: UndefinedClass - tests/system/CodeIgniterTest.php:141:33 - Class, interface or enum named Tests\Support\Controllers\Popcorn does not exist (see https://psalm.dev/019)
        $routes->set404Override('Tests\Support\Controllers\Popcorn::pop');

https://github.com/codeigniter4/CodeIgniter4/actions/runs/5067800693/jobs/9099299107?pr=7519

And I reran the check, it passed.

[kenjis]

@iRedds Thank you!