[BUGFIX] Require PHP version in ext_emconf.php

EXT: wv_deepltranslate requires at least PHP 7.4, as stated in its
`composer.json` file. However, if the TYPO3 instance is a
non-Composer-based installation, the extension will be installed
regardless of the used PHP version, leading to hard crashes.

To catch this case, the PHP version constraint is added to
`ext_emconf.php`.

Fixes: web-vision#170

ext_emconf.php

@@ -24,6 +24,7 @@
  'version' => '3.3.0',
  'constraints' => [
  'depends' => [
+ 'php' => '7.4.0-8.1.99',
  'typo3' => '9.5.1-11.5.99',
  ],
  'conflicts' => [],