Note: in Figma Design, when creating a new grid, the container and all rows and columns are automatically set to HUG, but in the Plugin API, new grids are FIXED and their tracks are FLEX.
Breaking change: previously, calling the GridTrackSize setters with a value would set the track to 'FIXED' if it was previously set as 'FLEX'. Now, the type will only be automatically set to 'FIXED' if the track was previously set to 'HUG'