Fix import path ITokenizer

[Borewit]

I think the import path should be

import {ITokenizer} from 'strtok3';

as the export of strtok3 is (updated):

"exports": {
    ".": {
      "node": "./lib/index.js",
      "default": "./lib/core.js"
    },
    "./core": "./lib/core.js"
  },

where both index.js & core.js export ITokenizer