fix: improve type for upload file v2

[WilliamBergamin]

Summary

This PR aims to fix #1846 by adding additional typing and related tests

Testing

1. Pull this branch
2. cd packages/web-api
3. npm build
4. In a project with the following app.ts

import { WebClient } from '@slack/web-api';

const client = new WebClient('mytoken');

(async () => {
  client.filesUploadV2({
    channel_id: 'C123',
    thread_ts: '173.029',
    content: 'test',
    filename: 'slack.json',
    title: 'test',
  });
})();

5. with npm install ~/path/to/node-slack-sdk/packages/web-api the above should not create any typescript errors or warnings
6. change channel_id -> channels, you should see the following warning when running the app

[WARN]  web-api:WebClient:0 Although the 'channels' parameter is still supported for smoother migration from legacy files.upload, we recommend using the new channel_id parameter with a single str value instead (e.g. 'C12345').

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[WilliamBergamin]

Not sure if we could do anything else then deprecate this 🤔

[filmaj]

Nailed it!

[filmaj]

Awesome! Nicely done. Hopefully sorting through all the different combinations of types/interfaces wasn't too bad 🙏

And don't worry about the node 22 failures - I am looking into it. I think it's a new npm bug. As long as 18 and 20 pass, you are good! Merge away!

[filmaj]

Nailed it!