what voltage level is a digital low for the interrupt mode LOW?
It would be great if I could set up an interrupt to turn on an LED
when the batteries were getting weak

That won't work. The thresholds for the pin registering LOW or HIGH
depend on the supply voltage. Very roughly:

a pin in guaranteed to register LOW if its voltage is below ⅓ Vcc
it is guaranteed to register HIGH if it's above ⅔ Vcc
nothing is guaranteed between ⅓ and ⅔ of Vcc.

For the exact voltage levels, you should check the datasheet.
Note also that those are guaranteed levels. The actual thresholds are
closer to ½ Vcc, with an hysteresis of roughly Vcc/10.
For detecting a low battery condition, you should use the ADC, combined
with either a voltage divider on Vcc or the internal bandgap reference.