Read tag and group descriptions from the plugin definition

Closes #3633005

Since 2.2.1, the MetaNameBase and GroupBase constructors read the description from $configuration['description']. It's stored in $plugin_definition['description'], so description() always returns nothing and the meta tag fields on the forms lost their own help text.

This reads it from the plugin definition again.

Best merged after or together with !279 (merged) (#3632998). Without it, a plugin that sets its description as a plain string instead of a translation would hit a TypeError, because description() is typed ?TranslatableMarkup.

This code was mainly generated by an AI with my guidance, I reviewed, tested, and refined it.