static @NonNull MutableContextSet of(@NonNull String key, @NonNull String value) {
        Objects.requireNonNull(key, "key");
        Objects.requireNonNull(value, "value");
        MutableContextSet set = create();
        set.add(key, value);
        return set;
    }
Creates a {@link MutableContextSet} from a context pair.

@param key   the key
@param value the value
@return a new MutableContextSet containing one context pair
@throws NullPointerException if key or value is null
api/src/main/java/net/luckperms/api/context/MutableContextSet.java