Add "token_strings" to the response of Tokenize

[mkozakov]

The API response of tokenize now includes a token_strings field. This PR adds it to the SDK.

Example:

const response = await cohere.tokenize({
  text: 'hello world'
});

console.log(response.body["tokens"]) // [33555, 1114]
console.log(response.body["token_strings"]) // ["hello", " world"]