Do not expose disabled element types

Problem/Motivation

A site builder can choose which Webform element types they want to enable in their website. We should only expose the ones that are enabled.

Steps to reproduce

1. Go to Structure > Webforms > Configuration > Elements (/admin/structure/webform/config/elements).
2. Expand the "Element types" fieldset.
3. Take note of one or more element types that are not enabled (e.g. CodeMirror).
4. Go to the GraphQL explorer.
5. Check that the corresponding type is not exposed in GraphQL (e.g. WebformElementWebformCodemirror).