Add CLI command to check if there's some version to be built

[mlocati]

What about adding a way to automatically build new concrete5 versions?

With this pull request, we can schedule a cron job like this:

composer run-script new-versions-available && \
    php ./vendor/bin/sami.php parse sami_config.php && \
    php ./vendor/bin/sami.php render sami_config.php

[mlocati]

I guess the cron job hasn't been implemented: at https://documentation.concrete5.org/api there's no APIs for 8.3.2...