Here's a free Game Jam License to DragonRuby Game Toolkit. 

The zip file includes a sample app/starting point specifically for this jam.

Here what the starter template looks like. It's got all the collision stuff in place and some gravity/physics. You can create a map live within the game, save it, and reload the map. Hopefully that's enough to get you started. I'm on Discord if you need any help.

Super Easy Setup Guide:

1. Unzip file.
2. Go to the directory that represents your operating system.
3. Run dragonruby.exe.
4. Open mygame/app/main.rb in the code editor of your choosing. A Metroidvania Starter template will load up.
5. Change the code.
6. Save the file.
7. Watch the game change.

How to Publish Your Game:

1. Done coding your game? Cool!
2. Fill out mygame/metadata/metadata.txt.
3. Double click dragonruby-publish.exe.
4. A folder will appear called "build" that includes Windows, Mac, Linux, and Web versions of your game.
5. Upload to Itch.

This is a web based code editor with a sample 2D platformer preloaded. Here is what it looks like.

1. Click link.
2. Press the Run Game! button to load up the code.
3. Change the code.
4. Ctrl+S to save and rerun game.