async getByARR(value, currency = 'usd') {
        // find a milestone of the ARR type by a given value
        return this.#store.find((item) => {
            return item.value === value && item.type === 'arr' && item.currency === currency;
        });
    }
@param {number} value
@param {string} [currency]

@returns {Promise<Milestone>}