feat: add Boot class and refactor index.php/spark/CodeIgniter::initialize()

[kenjis]

Description
Closes #7824

• move Kint loading 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
• add Boot class, then index.php, spark, and Test/bootstrap.php use it
• add Config\Optimize::$configCacheEnabled
• add Config\Optimize::$locatorCacheEnabled
• deprecate system/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

[kenjis]

I found that spark does not show back trace in production environment.
Is this intentional?

development:

(develop *$=)$ ./spark 

CodeIgniter v4.4.5 Command Line Tool - Server Time: 2024-02-19 10:33:30 UTC+00:00

[ParseError]
syntax error, unexpected token "protected", expecting "," or ";"
at SYSTEMPATH/Commands/ListCommands.php:38

Backtrace:
  1    SYSTEMPATH/Autoloader/Autoloader.php:253
       CodeIgniter\Autoloader\Autoloader()->includeFile('/Users/kenji/work/codeigniter/official/CodeIgniter4/vendor/composer/../../system/Commands/ListCommands.php')

  2    [internal function]
       CodeIgniter\Autoloader\Autoloader()->loadClassmap('CodeIgniter\\Commands\\ListCommands')

  3    SYSTEMPATH/CLI/Commands.php:105
       class_exists('CodeIgniter\\Commands\\ListCommands')

  4    SYSTEMPATH/CLI/Commands.php:46
       CodeIgniter\CLI\Commands()->discoverCommands()

  5    SYSTEMPATH/Config/Services.php:174
       CodeIgniter\CLI\Commands()->__construct()

  6    SYSTEMPATH/Config/BaseService.php:258
       CodeIgniter\Config\Services::commands(false)

  7    SYSTEMPATH/Config/BaseService.php:199
       CodeIgniter\Config\BaseService::__callStatic('commands', [...])

  8    SYSTEMPATH/Config/Services.php:171
       CodeIgniter\Config\BaseService::getSharedInstance('commands')

  9    SYSTEMPATH/Config/BaseService.php:258
       CodeIgniter\Config\Services::commands()

 10    SYSTEMPATH/CLI/Console.php:41
       CodeIgniter\Config\BaseService::__callStatic('commands', [])

 11    ROOTPATH/spark:102
       CodeIgniter\CLI\Console()->run()

production:

(develop *$=)$ ./spark 

CodeIgniter v4.4.5 Command Line Tool - Server Time: 2024-02-19 10:33:42 UTC+00:00

[ParseError]
syntax error, unexpected token "protected", expecting "," or ";"
at SYSTEMPATH/Commands/ListCommands.php:38

(develop *$=)$ 

[kenjis]

Since this PR was too large, I divided it into three parts: #8603, #8604, #8605