Turns anonymous functions into class methods

[flore2003]

Currently most methods on the MistralClient are implemented as anonymous functions/class fields (myMethod = function() {...}). This makes it hard to inherit, overwrite, and monkeypatch these methods.

Example:

import * as MistralAI from "@mistralai/mistralai";

const originalChat = MistralAI.prototype.chat;
MistralAI.prototype.chat = function(...) {...};
// originalChat is undefined

This PR changes all of these functions to real class methods, which allows for easier monkeypatching etc.

Closes #58