get(ruleId) {
		if (typeof this._rules[ruleId] === "string") {
			this.define(ruleId, require(this._rules[ruleId]));
		}
		if (this._rules[ruleId]) {
			return this._rules[ruleId];
		}
		if (builtInRules.has(ruleId)) {
			return builtInRules.get(ruleId);
		}

		return null;
	}
Access rule handler by id (file name).
@param {string} ruleId Rule id (file name).
@returns {Rule} Rule object.