CommonJS support for npm packages

All npm-published packages now support both ESM and CommonJS module
formats through tsdown dual package builds. This resolves the dual
package hazard that occurred when using Fedify with CommonJS-based
NestJS applications, eliminating the need for Node.js's
--experimental-require-module flag.

Changes include:

- Configure tsdown to build both ESM and CJS formats for npm packages
- Update package.json exports for dual package support
- Add proper type definitions for both module systems
- Update documentation to recommend ESM while noting CommonJS support

Closes https://github.com/fedify-dev/fedify/issues/429