feat: Create start/end node components

[lornakelly]

Description

Introduce dedicated Start and End node components in the editor. These nodes are not part of the Serverless Workflow specification or underlying model, as they do not represent executable tasks. Instead, they serve as visual aids within the diagram to clearly indicate the entry and exit points of a workflow, making the overall structure easier to understand at a glance.

Motivation

Improve diagram readability and user orientation by clearly indicating workflow entry and exit points.

Proposed Implementation

No response

Definition of Done

• Implementation: Fully implemented according to the Serverless 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.