function cellToVertex(h3Index, vertexNum) {
    const [lower, upper] = h3IndexToSplitLong(h3Index);
    const vertexIndex = C._malloc(SZ_H3INDEX);
    try {
        throwIfError(H3.cellToVertex(lower, upper, vertexNum, vertexIndex));
        return validateH3Index(readH3IndexFromPointer(vertexIndex));
    } finally {
        C._free(vertexIndex);
    }
}
Find the index for a vertex of a cell.
@static
@param {H3IndexInput} h3Index     Cell to find the vertex for
@param {number} vertexNum         Number (index) of the vertex to calculate
@return {H3Index}     Vertex index
@throws {H3Error}     If the input is invalid