feat: Create the language-service package #329

Description

Language service spike: #209

Create packages/language-service (@openworkflowspec/language-service), based on @volar/language-service, to provide the common foundation for the JSON and YAML language services.

Motivation

Provide the common, editor-agnostic package foundation required to implement Open Workflow-specific authoring assistance.

Proposed Implementation

Set up the base @volar/language-service dependency and the package infrastructure required by the JSON and YAML language service implementations.

JSON and YAML support will be implemented separately in:

• feat: Implement volar-service-json for JSON language support #321
• feat: Implement volar-service-yaml for YAML language support #322

The package should remain private for now. Publishing it to npm will be handled in a separate task.

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.