build(editor): halkyon_editor joins the workspace (ADR 0033, D37)

The package for the general parts of Learn's editor, its strings first;
the code moves next.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_editor/lib/halkyon_editor.dart

@@ -0,0 +1,3 @@
+/// Halkyon's rich text: the editor, the read-only view and the document
+/// they keep.
+library;