Add a new `Process.cwd` static method that retrieves the current working directory via libuv's `uv_cwd` function. The implementation includes proper error handling that aborts the fiber with a descriptive message on failure. The foreign method is registered in the module table and exposed in the Wren API with a foreign static declaration. A test suite validates that the returned value is a non-empty string starting with "/", resolves correctly via `File.realPath`, and corresponds to an existing directory.
