feat: add typescript support / tsup / rebuild core script / refactor / change project name

[dziksu]

Hi, this pull request introduces TypeScript support to the Walkthrough.js project.

It adds a TypeScript build configuration using tsup
to provide better type safety, developer experience, and maintainability.

What has been changed:

• Changed project name into "@walkthroughjs/core" - for the future, this could be a better way to deliver certain adapters, such as "@walkthroughjs/vue" or "@walkthroughjs/react"
• Rewrite /src to typescript
• Added TypeScript configuration (tsconfig.json)
• Integrated tsup for bundling and type generation
• Updated build scripts in package.json
• Ensured compatibility with existing JavaScript codebase
• Preserved original functionality and export structure

Important:

This change does not break existing JavaScript usage.
Developers can continue using the library as before, while benefiting from type definitions when using TypeScript.

Feel free to test and made some changes,
Cheers