_cacheNumberFormats() {
        // Load the standard number format codes into the caches.
        this._numberFormatCodesById = {};
        this._numberFormatIdsByCode = {};
        for (const id in STANDARD_CODES) {
            if (!STANDARD_CODES.hasOwnProperty(id)) continue;
            const code = STANDARD_CODES[id];
            this._numberFormatCodesById[id] = code;
            this._numberFormatIdsByCode[code] = parseInt(id);
        }

        // Set the next number format code.
        this._nextNumFormatId = STARTING_CUSTOM_NUMBER_FORMAT_ID;

        // If there are custom number formats, cache them all and update the next num as needed.
        this._numFmtsNode.children.forEach(node => {
            const id = node.attributes.numFmtId;
            const code = node.attributes.formatCode;
            this._numberFormatCodesById[id] = code;
            this._numberFormatIdsByCode[code] = id;
            if (id >= this._nextNumFormatId) this._nextNumFormatId = id + 1;
        });
    }
Cache the number format codes
@returns {undefined}
@private