fix: make dist/index.js run as CommonJS while keeping source as ESM

The root package.json keeps "type": "module" so the source file
(deployment-frequency-event.js) can use ESM import syntax for local
development and testing.

The dist/package.json is changed from {"type": "module"} to
{"type": "commonjs"} so that dist/index.js (the ncc-bundled output
referenced by action.yml) is treated as CommonJS by Node.js. This is
required because GitHub Actions node20 runner loads the entry point
via require(), which cannot load ES modules.

The dist/index.js is rebuilt by ncc as pure CommonJS (IIFE-wrapped,
uses require(), exports via module.exports).

Author: pannago