Add PHP version and required/optional extensions to composer.json

There are a few changes that could be done in composer.json file to improve following:

1. let PhpStorm and other tools know what PHP version Moodle supports - at the moment you need to configure them manually
2. let PhpStorm and other tools know what PHP extensions are required - at the moment PhpStorm shows warnings (for example any json_encode call)
3. admins/developers cannot find Moodle requirements easily, they have to read environment XML file
4. Moodle developer running "composer update" to create composer.local needs to be using lowest support PHP version

Solution is to:

• add PHP version requirement to composer.json
• add optional and required PHP extensions to composer.json
• add platform setting to composer to get composer.lock file for all PHP binaries

Benefits:

1. PhpStorm can show unavailable PHP syntax based on PHP versions required by Moodle
2. PhpStorm does not show warnings about missing PHP extensions
3. Developers are notified about missing PHP extensions and unsupported PHP versions when they init behat or phpunit
4. Moodle developers can use any PHP version when building composer.lock