feat: adds todos for exercises 2, 4, 5 and 6

[VineethK336]

Hello Tyler! I went through you React Performance course & it was really great! Thanks a lot for keeping it open & free for everyone!

Why this PR?

• I felt that if there was a branch where learners could update code as they walkthrough the lessons, it would be great.
• Right now, the master branch is optimized (except for Suspense lesson).

Let me know what you think. Feel free to comment or edit the PR if you'd like. It's also fine if you don't have time or intent to make any changes.🙂

If you would like to go ahead with the PR:

1. I would request you to create an equivalent exercises(or any better name) branch in your repo which is in sync with master so that PR base branch can be changed to that new branch & keep master as is so that it would serve as a reference.
2. I have todos for the rest of the exercises as well locally. Would create a follow up PR(for smaller PRs) or push commits into same PR branch.