feat: add CI check to require version bump when package files change

Adds a check-version-bump job that runs on MRs and branch pushes to ensure
that when package-related files (src/, package.json, webpack.config.js,
tsconfig.json) are modified, the package version must also be bumped.

This prevents merging changes that would result in an outdated published
package.

Closes #5

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>