refactor(learn): the editor's general parts move to halkyon_editor (ADR 0033, D37)

Todo stores editor documents too, so the document, the editor, its view,
the format bar, links, the clipboard, scripts and EditorBlocks move, with
their strings and tests, to a shared package on the same appflowy_editor
fork. What is Learn's alone stays in Learn on top of it, and what the
editor took from it directly is inverted: EditorBlocks gains textMarks,
formatActions and onEditorClosed, RichContentEditor a wrapEditor, and an
InsertableBlock's name takes the BuildContext. Learn's screens use
LearnContentEditor and LearnContentView, which add Learn's blocks and the
grammar check. Learn behaves as before.

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

apps/learn/halkyon_learn_flutter/pubspec.yaml

@@ -79,6 +79,10 @@ dependencies:
   # run their work in (SerialQueue, CoalescingRunner).
   halkyon_core:
     path: ../../../packages/halkyon_core
+  # Rich text's general parts (a shared package, ADR 0033, D37); Learn's own
+  # blocks build on it in lib/src/core/editor/.
+  halkyon_editor:
+    path: ../../../packages/halkyon_editor
   halkyon_learn_client:
     path: ../halkyon_learn_client
   # Language models' APIs (a shared package, ADR 0021, D15): the protocols a