feat(api): export {tokenize} function

[azu]

Note: backward compatibility for <= 1.1.0

kuromojin v1.1.0 export tokenize as default function

import kuromojin from "kuromojin";
// kuromojin === tokenize

Recommended use import {tokenize} from "kuromojin" instead of it

import {tokenize} from "kuromojin";