Add TypeScript declaration file

[rsweetland]

Add declarations for all method and class names for fun and easy TypeScript development in other libs.

[sebestindragos]

After spending a lot of time on trying to find a proper way to write the export in the declaration file, I decided to actually change the source code.

Right now, the sdk exports a class as the whole export object via:
module.exports = MailBotsClient

This pattern is quite weird to use in typescript as it requires either a special way of importing the module
import thing = require("./Thing");
which works fine until you want to use that import as a type

or to enable a compiler flag (esModuleInterop) and use default import style syntax:
import MailBotsClient from "@mailbots/mailbots-sdk";, but this leads to other compiling issues.

The easiest fix is to export a named export from the mailbots-sdk.js module:

module.exports = {
  MailBotsClient
};

and use standard typescript module syntax.
More details can be found in (this article)[https://dzone.com/articles/import-statements-in-typescript-which-syntax-to-us].

Need to remember that after we integrate this branch we will have breaking changes in code that uses this sdk.