feat(character): restrict jumping to when on a platform

Change the jump() condition from checking velocity to detecting whether the character's feet are touching a platform. This prevents mid-air jumps and makes jump behavior more intuitive for platformer gameplay.

README.md

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+## How to run
+
+Simply run:
+```python
+python3 -m http.server 8081
+```
+
+Inside of the project's cloned folder