feat: support ".finally()"

[kettanaito]

• Adds support for .finally() method on the deferred promise.
• Fixes an issue when catching an exception would still treat the deferred promise as unhandled (re-assign this.promise in the then/catch/finally methods).
• Adds JSDoc descriptions for the .then(), .catch(), and .finally() methods.

[kettanaito]

Released: v1.1.0 🎉

This has been released in v1.1.0!

• 📄 Release notes
• 📦 npm package

Make sure to always update to the latest version (npm i @open-draft/deferred-promise@latest) to get the newest features and bug fixes.

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