Add regions.json for cell segmentation

[benneely]

ihc-analysis-pipeline expects a regions.json file to be present in mm_e16.5_20x_sox9_sftpc_acta2/light_color_corrected; however, there isn't one. If one tries to copy the regions.json file from the outer folder, this doesn't work because it's expecting a different format. This may already be done, but can't find in any documentation.