Update Blesta Composer Installer for Composer 2.0 compatibility

[rjocoleman]

This PR updates the installer to work with Composer 2.0, as Composer 1.x support is being discontinued soon.

I'm new to Blesta, so unsure if this is even needed any more but I wanted to evaluate a Composer based management workflow and this was a blocker.

Changes:

• Updated to Composer 2.0+ (from 1.x)
• Raised PHP requirement to 8.1+ (from 5.3)
• Refactored to use Composer plugin architecture
• Replaced Travis CI with GitHub Actions
• Updated PHPUnit and added PHPStan for static analysis
• Added support for messenger components
• Updated coding standards to PSR-12

Breaking Changes:

• Requires PHP 8.1+
• Requires Composer 2.0+
• Update package requirement to ~2.0 in composer.json

Testing:

• Tests updated to modern PHPUnit and pass on PHP 8.1-8.4
• Coveralls is untested

$ vendor/bin/phpunit --coverage-text               

PHPUnit 10.5.46 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.4.6 with PCOV 1.0.12
Configuration: ./composer-installer/phpunit.xml.dist

.................                                                 17 / 17 (100%)

Time: 00:00.056, Memory: 14.00 MB

OK (17 tests, 73 assertions)


Code Coverage Report:
  2025-05-28 07:50:26

 Summary:
  Classes: 100.00% (2/2)
  Methods: 100.00% (7/7)
  Lines:   100.00% (65/65)

Blesta\Composer\Installer\Installer
  Methods: 100.00% ( 3/ 3)   Lines: 100.00% (  6/  6)
Blesta\Composer\Installer\InstallerPlugin
  Methods: 100.00% ( 4/ 4)   Lines: 100.00% ( 59/ 59)