Add lit-element to create-app supported templates

Editor assessment

The subject `docs: add lit-element to create-app supported templates` names the framework, the tool, and the exact list extended, which is a complete documentation statement. The docs type is right, and the create-app naming substitutes for a scope. No body or footer appears, acceptable because the subject fully describes the addition. No junk, no trailing period, sensible length. A reader knows exactly which template list gained an entry.