fix: move Kint loading to Autoloader

[kenjis]

Description
Supersedes #8558

• move CodeIgniter::initializeKint() to Autoloader
  • CodeIgniter::initializeKint() registers an autoloader. So it is better to be in Autoloader.
  • Config caching cannot load Kint files. See https://forum.codeigniter.com/showthread.php?tid=88992&pid=414620#pid414620

The following items are also needed, so included in this PR:

• move CodeIgniter::bootstrapEnvironment() to bootstrap.php
• move Services::exceptions()->initialize() to bootstrap.php
• move CodeIgniter::resolvePlatformExtensions() to bootstrap.php

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

[github-actions]

👋 Hi, @kenjis!

We detected conflicts in your PR against the base branch 🙊
You may want to sync 🔄 your branch with upstream!

Ref: Syncing Your Branch

[kenjis]

This is the first PR of a story.
See the next #8604, #8605, and #8610.

[MGatner]

Won't this always be defined because it was required for the Autoloader?

public $psr4 = [
    APP_NAMESPACE => APPPATH, // For custom app namespace
    'Config'      => APPPATH . 'Config',
];

[kenjis]

It is defined in app/Config/Constants.php.