Convert explorable-explanation package to TypeScript

[mohdsayed]

Task

Description:
The @google-psat/explorable-explanation package is currently written in vanilla JavaScript, while all other packages in the project use TypeScript. To ensure consistency, improve type safety, and enhance maintainability, we should refactor this package to use TypeScript.

Related Issue:
#857

Acceptance Criteria:

• The package should be fully type-safe.
• No functionality should break during the transition.
• The package should integrate seamlessly with other TypeScript-based packages in the project
• Improvement: Convert explorable-explanations package to typescript #940 (review)