Check composer platform requirements

If you ever need to check what php version do composer packages and their dependencies require, there's a special command just for that.

```bash
composer check-platform-reqs
```

The output should look something like below. Maybe without the failed steps.

```bash
ext-dom        20031129                                                success  
ext-fileinfo    7.3.19                                                  success  
ext-filter      7.3.19                                                  success  
ext-json       1.7.0                                                   success  
ext-libxml     7.3.19                                                  success  
ext-mbstring   7.3.19                                                  success  
ext-openssl    7.3.19                                                  success  
ext-pcre       7.3.19                                                  success  
ext-Phar       7.3.19                                                  success  
ext-SimpleXML  7.3.19                                                  success  
ext-tokenizer  7.3.19                                                  success  
ext-xml        7.3.19                                                  success  
ext-xmlwriter  7.3.19                                                  success  
lib-pcre       10.32                                                   success  
php            7.3.19    league/commonmark requires php (^7.4 || ^8.0) failed   
php            7.3.19    league/config requires php (^7.4 || ^8.0)      failed   
```

This is useful when your app fails due to unfulfilled dependency requirements.

Zeno Popovici
12 Aug 2021