feat: Add FetchMocker#mockObject() method to mock non-global fetch

[nzakas]

This pull request introduces new functionality to mock the fetch function on specific objects, enhancing the flexibility of the FetchMocker class. The most important changes include updates to the documentation, the addition of new methods to the FetchMocker class, and corresponding tests.

Enhancements to FetchMocker functionality:

• src/fetch-mocker.js: Added mockObject and unmockObject methods to allow mocking and unmocking the fetch function on specific objects. These methods store the original fetch functions in a WeakMap and replace them with the mocked fetch function.

Documentation updates:

• docs/src/content/docs/fetch-mockers/creating-fetch-mocker.mdx: Updated documentation to include examples and explanations of how to use the new mockObject and unmockObject methods to mock the fetch function on specific objects.

Testing updates:

• tests/fetch-mocker.test.js: Added tests for the new mockObject and unmockObject methods, verifying their functionality, including handling custom property names and restoring multiple mocked properties.

Codebase adjustments:

• src/fetch-mocker.js: Updated the mockGlobal and unmockGlobal methods to use the new mockObject and unmockObject methods for consistency and code reuse.
• src/fetch-mocker.js: Changed the documentation comment for the fetch property to reflect that it is the created fetch function, not the global fetch function.