feat(api-docs-spectral-linter): add new frontend system support

[Tusharkshahi]

What

Adds support for the new frontend system to the api-docs-spectral-linter plugin.

Closes #44

Changes

• New /alpha entry point (src/alpha.tsx) with:
  • ApiBlueprint for the Spectral Linter API
  • EntityContentBlueprint for the Spectral Linter tab on API entity pages
  • Entity filter that automatically shows the tab only on openapi and asyncapi API entities
• Updated package.json with /alpha export paths and @backstage/frontend-plugin-api dependency
• Added test for the new entry point
• Changeset included (minor bump)

Backward Compatibility

The existing / entry point is unchanged. Consumers using the old frontend system are unaffected.

Usage (new frontend system)

import spectralLinterPlugin from '@dweber019/backstage-plugin-api-docs-spectral-linter/alpha';

export default createApp({
  features: [spectralLinterPlugin],
});