is it safe to use macbook air while charging

Plug in as often as you can. Your Mac will manage it's battery health without your intervention:

About battery health management in Mac notebooks - Apple Support

Plug in the computer whenever convenient to do so. Leaving the computer plugged in when using it will not damage the computer or the battery.

Operating while connected to a power adaptor IS a designed-in normal operating mode.

Yes it certainly is. Just ensure it does not overheat.