Real composer scripts

[morawskim]

This PR closes #901

At the moment in recipe we can use composer-scripts, but those commands will be running only during composer install or composer update.

This PR adds a new configuration option composer-commands. We can add to recipe:

.....
"composer-commands": {
    "command": "vendor/bin/foo"
}

and flex will add to composer.json necessary entry and we will able to call composer run command to execute vendor/bin/foo.

This behavior is very similar to current Makefile configurator as mentioned in #901.

[morawskim]

I will fix the build "PHP 7.4 low-deps" at weekend.