Add alert for low oxygen.

Resources/Textures/Interface/Alerts/breathing.rsi/meta.json

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+{"version": 1, "size": {"x": 32, "y": 32}, "license": "CC-BY-SA 3.0", "copyright": "https://github.com/tgstation/tgstation/blob/2f18e6232ce5897554351194082469a043c5ab06/icons/hud/screen_alert.dmi", "states": [{"name": "not_enough_co2", "directions": 1, "delays": [[0.5, 0.5]]}, {"name": "not_enough_nitro", "directions": 1, "delays": [[0.5, 0.5]]}, {"name": "not_enough_oxy", "directions": 1, "delays": [[0.5, 0.5]]}, {"name": "not_enough_tox", "directions": 1, "delays": [[0.5, 0.5]]}, {"name": "too_much_co2", "directions": 1, "delays": [[0.5, 0.5]]}, {"name": "too_much_nitro", "directions": 1, "delays": [[0.5, 0.5]]}, {"name": "too_much_oxy", "directions": 1, "delays": [[0.5, 0.5]]}, {"name": "too_much_tox", "directions": 1, "delays": [[0.5, 0.5]]}]}