feat: adding Learn Intermediate OOP by Building a Platformer Game

[jdwilkin4]

Link to live example

https://u57qxn.csb.app/

Link to codesandbox

https://codesandbox.io/s/fcc-platformer-u57qxn?file=/index.html:0-881

Checklist:

• [x] I have read freeCodeCamp's contribution guidelines.
• [x] My pull request has a descriptive title (not a vague title like Update index.md)

[naomi-lgbt]

I've got a few more suggestions coming - this is looking really good, just some UX nitpicks.

[jdwilkin4]

@naomi-lgbt

I like these new changes 👍