feat: Create the dual-editor package with resizable split-view layout

[fantonangeli]

Description

Create packages/dual-editor (@openworkflowspec/dual-editor) with a resizable split-view containing the Text Editor and Diagram Editor.

Motivation

Provide the package foundation and layout required to use the Text Editor and Diagram Editor side by side.

Proposed Implementation

• Create the @openworkflowspec/dual-editor workspace package.
• Depend on @openworkflowspec/text-editor and @openworkflowspec/diagram-editor.
• Add a resizable split-view with a draggable divider between the two editors.
• Keep the package private for now. Publishing will be handled separately.

Definition of Done

• Implementation: Fully implemented according to the Open Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.