add php extension validation

[ejnshtein]

Prevents those errors:

Unexpected error during composer install.

Error: Code: 2
        Response: No composer.lock file present. Updating dependencies to latest instead of installing from lock file. See https://getcomposer.org/install for more information.
Loading composer repositories with package information
Info from https://repo.packagist.org: #StandWithUkraine
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires magento/product-community-edition 2.4.3-p1 -> satisfiable by magento/product-community-edition[2.4.3-p1].
    - magento/product-community-edition 2.4.3-p1 requires ext-iconv * -> it is missing from your system. Install or enable PHP's iconv extension.

To enable extensions, verify that they are enabled in your .ini files:
    - /home/suresh/magento/magento2/node_modules/.create-magento-app-cache/php.ini
    - /home/suresh/.phpbrew/php/php-7.4.27/var/db/gd.ini
    - /home/suresh/.phpbrew/php/php-7.4.27/var/db/intl.ini
    - /home/suresh/.phpbrew/php/php-7.4.27/var/db/libsodium.ini
You can also run `php --ini` in a terminal to see which files are used by PHP in CLI mode.
Alternatively, you can run Composer with `--ignore-platform-req=ext-iconv` to temporarily ignore these required extensions.

Those errors are appearing because some users decide to compile PHP themselves using PHPBrew, but their version of PHP is missing some important bundled extensions.
So, we need to tell them that magento-scripts should handle this.