Here's what I've found:
./editor/code_editor.h is used by ./editor/plugins/script_text_editor.h
That is then used in ./editor/plugins/script_editor_plugin.h
In ./scene/register_scene_types.cpp
I tried registering CodeTextEditor into ClassDB. It compiled fine, however the editor just crashed on launching a project. So I tried registering ScriptTextEditor into ClassDB instead and got the same crash.
I've also noticed that ScriptTextEditor and CodeTextEditor are lacking bind_methods(). I think to get this working, one would need to implement a bind_methods() and define the default properties.. It's just a hunch though! Good luck, and I will update on this post if I find anything else out!
Here is the backtrace from the crash I got with the last attemp:
[0] mtx_do_lock (d:\agent_work\63\s\src\vctools\crt\github\stl\src\mutex.cpp:92)
[1] mtx_do_lock (d:\agent_work\63\s\src\vctools\crt\github\stl\src\mutex.cpp:92)
[2] EDITOR_GET
[3] CodeTextEditor::CodeTextEditor
[4] ScriptTextEditor::ScriptTextEditor
[5] ClassDB::creator<ScriptTextEditor> (C:\godot\core\class_db.h:143)
[6] ClassDB::instance
[7] ClassDB::class_get_default_property_value
[8] DocData::generate
[9] DocData::generate
[10] EditorHelp::generate_doc
[11] EditorNode::EditorNode
[12] Main::start (C:\godot\main\main.cpp:1808)
[13] widechar_main
[14] main
[15] main
[16] __scrt_common_main_seh (d:\agent_work\63\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288)
[17] BaseThreadInitThunk