refactor(commands): organize command definitions into modular commands/ directory #3895

Description

Organize command definitions into a dedicated commands/ directory structured by feature/domain instead of keeping all actions inside a single command_palette_actions.dart file.

Originated from discussion in #3870.

Motivation

As the number of commands in DartPad grows (pub commands, workspace actions, editor actions, panel toggles), a single file will become difficult to maintain. Splitting them into focused files improves discoverability and testability.

Proposed Changes / Acceptance Criteria

• Create a dedicated directory: lib/features/shared/commands/ (or feature-specific directories where appropriate).
• Split commands by domain:
  • commands/pub_commands.dart (Pub get, outdated, upgrade, etc.)
  • commands/editor_commands.dart / workspace_commands.dart
  • commands/commands.dart (central aggregator exporting allCommandActions)
• Use final instead of const for action declarations where closures or dynamic context bindings are needed.