fix: improve package exports and module resolution

[chybisov]

Why was it implemented this way?

Improves package compatibility for consumers by updating the export structure in formatPackageJson.js:

• default → import in export conditions — makes the ESM intent explicit and improves resolution in bundlers that differentiate between import and require conditions
• Added module field — provides a fallback entry point for older bundlers (e.g. Webpack 4) that don't support the exports map
• Added typesVersions — ensures TypeScript can resolve subpath export types in projects using moduleResolution: "node" (not just "bundler" or "node16")
• Restructured skeleton export — moved the ./skeleton subpath into the shared allSubpathExports so it's included in typesVersions generation

Also includes the 4.0.0-alpha.3 version bump across all packages.

Checklist before requesting a review

• I have performed a self-review and testing of my code.
• This pull request is focused and addresses a single problem.
• If this PR modifies the Widget API or adds new features that require documentation, I have updated the documentation in the public-docs repository.