Add extension key to composer.json to mitigate composer warning

[peterkraume]

Beginning with version 3.1.0 of typo3/cms-composer-installers the package emits a warning:

TYPO3 Extension Package "mittwald/web2pdf", does not define extension key in composer.json. Specifying the extension key will be mandatory in future versions of TYPO3 (see: https://docs.typo3.org/m/typo3/reference-coreapi/master/en-us/ExtensionArchitecture/ComposerJson/Index.html#extra)

This patch mitigates the problem.