Update Third-Party-Library Horde Text Diff

Update the third party library horde text diff to the latest version for PHP 7.4. compatibility.

Using therefore the composer pear repository "https://pear.horde.org" and package "pear-pear.horde.org/Horde_Text_Diff": "^2.2"

The composer usage is also changed. The composer.lock is now also under version control for composer install usage.