Add highway-env to third party environments in environments.md (#1839)
Add highway-env to third party environments in environments.md (#1839)

[highway-env](https://github.com/eleurent/highway-env) is a simulated
driving environment for high-level decision-making.

Co-authored-by: pzhokhov <3fddac958924aef220f202ca567388ddab3f14a8@openai.com>
Farama-Foundation/Gymnasium,Farama-Foundation/Gymnasium