avoid passing around reference of cached tokens

[PatrickElfert]

The tokenize function was previously returning a reference to the cached tokens. This caused issues when mutating the array and then accessing the cache, resulting in different outputs for the same input text. To resolve this, I've added logic to create a shallow copy of the tokens before returning them, ensuring the cached tokens are not inadvertently mutated.

[azu]

This approach does not prevent the mutate itself of each token.

const tokens = await tokenize(...);
tokens[0].word_id = "..."; // affect to cache

I think we should avoid mutating KuromojiToken. (The design was not intended to mutate it.)
Therefore, I feel that changing Promise<KuromojiToken[]> in the return value to Promise<Readonly<Readonly<KuromojiToken>[]>> is a compromise. (We also need to write it to documentation)

Freezing the return Object.freeze(value) is expensive, and storing a copy of the token in the cache can also be expensive/incomplete.
Therefore, I think it is better to solve this problem with just a type.

Related:

• Freezing at compile time | Structura.js