fix: Ensure Response#mode is 'cors' when Request#type is 'cors'

[nzakas]

This pull request ensures that the Response#mode property is properly set to "cors" when the type property of the Request is set to "cors". This also ensures that the default Response#mode is "cors" per the Fetch API specification (Bun sets it to "default".

The most important changes include setting the mode property correctly in the createCustomRequest function, updating the FetchMocker class to set response properties, and modifying the MockServer tests to validate response status and type.

Replaces and closes #57.

Improvements to fetch API handling:

• src/custom-request.js: Added logic to set the mode property to "cors" by default if not specified, ensuring consistency across different runtimes.
• src/fetch-mocker.js: Updated the FetchMocker class to set the url and type properties of the response based on the request's mode.

Updates to test cases:

• tests/custom-request.test.js: Added a test case to verify that the default mode of a CustomRequest is "cors".
• tests/fetch-mocker.test.js: Added test cases to ensure that the response type is set correctly based on the request's mode.
• tests/mock-server.test.js: Modified existing tests to validate response status and type instead of the URL, ensuring comprehensive coverage of the new behavior. [1] [2] [3] [4] [5]