The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope. I. Instrument Overview and In-flight Performance - Doyon, René; Willott, Chris J.; Hutchings, John B.; Sivaramakrishnan, Anand; Albert, Loïc; Lafrenière, David; Rowlands, Neil; Begoña Vila, M.; Martel, André R.; LaMassa, Stephanie; Aldridge, David; Artigau, Étienne; et al. (Publications of the Astronomical Society of the Pacific, 2023)