Will the 1/2-inch line now feeding the water heater have enough capacity to support
both the water heater and a gas range?

No. Run a new line from the source to the range.... the source being
at least a 3/4" line, but preferrably a 1" line.

You don't want your water heater to have a drop in volume (and
pressure, but volume is important), causing a low flame or for the
pilot to go out. The smaller the line, the less volume the same
pressure will move vs the larger the line, the more volume the same
pressure will move.

You said it right, "capacity", meaning volume, as opposed to
pressure. The source (your meter manifold) has a regulator to
automatically maintain constant pressure.

Sonny