feat!: new RESTClient

[HeadTriXz]

This pull request deprecates all REST methods of Client and moves them into a new class called RESTClient. This will allow for making requests without needing to create an instance of Client.

In addition, this PR introduces a completely new RequestHandler, which will handle requests made through RESTClient. The new RequestHandler provides better performance and less spaghetti code 🍝.

The implementation of RESTClient has been designed with full backwards compatibility in mind, so existing code should continue to work seamlessly. One significant improvement in this PR is the use of undici's fetch function for making HTTP requests.

As this is a significant change, extensive testing will be required to ensure its stability and correctness. ~~It also heavily depends on #46. Therefore, this PR will remain a draft until all testing is completed and #46 is merged.~~

Please review the changes and provide feedback as needed.

~~The new files currently use a different coding style (see class fields) than the rest of the library. It may, or may not change in the future, depending on #46.~~