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/l10n.yaml

@@ -0,0 +1,14 @@
+# Flutter's gen-l10n: the strings in lib/l10n/*.arb become
+# HalkyonEditorLocalizations, generated into lib/l10n/halkyon_editor_localizations*.dart
+# (committed, never edited or formatted by hand, as halkyon_ui's).
+# halkyon_editor_en.arb, the template, is British English, also the fallback;
+# halkyon_editor_en_US.arb overrides what American English spells differently;
+# halkyon_editor_en_GB.arb is empty; German says "du".
+arb-dir: lib/l10n
+template-arb-file: halkyon_editor_en.arb
+output-dir: lib/l10n
+output-localization-file: halkyon_editor_localizations.dart
+output-class: HalkyonEditorLocalizations
+# `HalkyonEditorLocalizations.of(context).foo` without a `!`: apps install the
+# delegate.
+nullable-getter: false