feat(plugin:duration): Add duration.asObject() method

[bhavyajain-glitch]

Hi maintainers,

This PR adds the new duration.asObject() method, as requested in issue #2971.

Closes: #2971

Summary Adds the asObject() method to the Duration class in src/plugin/duration/index.js.

Updates types/plugin/duration.d.ts with a new DurationAsObject interface and the asObject() method.

Adds a new test suite to test/plugin/duration.test.js to validate the new method.

This change passes all npm run lint and npm test checks and maintains 100% test coverage.

Happy to make any adjustments!