Write Tests in JavaScript

[threeal]

This issue proposes a shift to writing testing files in JavaScript for this template. Doing so has several advantages, making the examples in the template simpler:

• Eliminates the dependency on ts-jest.
• Enables direct testing of distribution files instead of solely the source files.
• Removes the separation between the main tsconfig and ESLint's tsconfig.

Moreover, testing doesn't inherently require TypeScript, as the package should be usable with or without TypeScript. Therefore, the testing should reflect the evaluation of the JavaScript part of the code.