refactor: clean up some code and added comments

[uragirii]

Your youtube video is amazing and a great start to a new type of content in Indian Dev Youtube space. Keep doing the good work! I refactored some of the code and to make it more production ready. I understand that this project is meant for beginners and not for production but still doesn't hurt to improve code a bit ;) Some changes:

• Added a README : it is helpful for beginners to read about project before getting into code. Its still a WIP, feel free to add more commands and instructions. Maybe even timestamps for different projects.
• Moved secrets to env files: A big oopsie. Also added dotenv package to read .env files. Added .env.sample file. If you merge https://github.com/hkirat/vercel/pull/4 I can rebase and remove those changes
• Cleaned up package.json by moving @types/* packages as dev dependencies
• Support for more content types: by using mime type package. (Already open by https://github.com/hkirat/vercel/pull/6)
• Used streams : Refactored code to use streams instead of reading whole files. I understand that reading files are more beginner friendly but we should also teach them how to use stream. (Also you already used streams one place so great!)
• Added a lot of comments for users to follow the code. Could be useful if you can add timestamps also ;)
• Added chunkedPromiseAll : to process promises chunk by chunk. This function might be quite advanced for beginners, if you want I can remove it.

If you merge other PRs with similar fixes, I will rebase the code/ remove my changes.

Feel free to push edit to this branch and leave comments. And again, thank you for a new start and keep making videos.

[MohakBajaj]

@uragirii Thanks for the useful changes. It's gonna help me review the code faster and add more features.