Add option to specify version in add lines requires

[HypeMC]

Currently, when requiring symfony/panther, Flex adds the following two lines to phpunit.dist.xml:

<extensions>
+    <bootstrap class="Symfony\Component\Panther\ServerExtension" />
+    <extension class="Symfony\Component\Panther\ServerExtension" />
</extensions>

This isn't entirely correct: the extension tag is used for PHPUnit <10, while bootstrap is used for PHPUnit >=10.

This PR adds support for specifying the required package version for each line, so only the appropriate one is added.

Recipes PR: symfony/recipes#1422

[fabpot]

Thank you @HypeMC.