Finally, the wait is over and there’s an editor which understands yacas syntax. Actually, not just an editor, this is Visual Studio Code, hands down the best development environment ever!
To try it out just download
yacas-syntax-highlighting-1.0.0.vsix from GitHub, run
code --install-extension yacas-syntax-highlighting-1.0.0.vsix
and enjoy writing yacas code in style!