[2.3] [Re-build] Manager Theme: Revert normal font for textareas due to many incompatibilities

This PR reverts the code introduced in https://github.com/modxcms/revolution/pull/11617 that makes the text inside textareas display in normal font. There are many textareas in extras now not displaying code correctly, so I changed the code to only affect (e.g. display normal font) in textareas with name attribute = "description" or "introtext" which still satisfies the initial issues behind this change. All other textareas display codefonts again.

In addition there is a new class .modx-text-content to apply for extra autors, if a textarea in their code needs to display normal text styles. The class of .modx-code-content is still available to force an input field to display the text in codefonts.