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>

packages/halkyon_editor/test/l10n/arb_files_test.dart

@@ -56,7 +56,10 @@ void main() {
 
   test("the English variants override the template's messages only, with "
       'the same placeholders', () {
-    for (final variant in ['halkyon_editor_en_GB.arb', 'halkyon_editor_en_US.arb']) {
+    for (final variant in [
+      'halkyon_editor_en_GB.arb',
+      'halkyon_editor_en_US.arb',
+    ]) {
       for (final MapEntry(:key, :value) in _messages(variant).entries) {
         expect(template, contains(key), reason: '$variant: $key');
         expect(

packages/halkyon_editor/test/l10n/halkyon_editor_localizations_test.dart

@@ -0,0 +1,21 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_editor/halkyon_editor.dart';
+import 'package:material_ui/material_ui.dart';
+
+void main() {
+  test('each language speaks its own, American English only where it '
+      'differs', () {
+    final british = lookupHalkyonEditorLocalizations(
+      const Locale('en', 'GB'),
+    );
+    final american = lookupHalkyonEditorLocalizations(
+      const Locale('en', 'US'),
+    );
+    final german = lookupHalkyonEditorLocalizations(const Locale('de'));
+
+    expect(british.formatAlignCenter, 'Centre');
+    expect(american.formatAlignCenter, 'Center');
+    expect(american.formatBold, british.formatBold);
+    expect(german.formatBold, isNot(british.formatBold));
+  });
+}