Add separate `f_esc` parameters for escape from HII regions and unattenuated light

[christopherlovell]

Right now in the pacman model f_esc controls escape from photoionisation regions as well as light that remains unattenuated by ISM dust. We should have two parameters controlling both of these.

https://github.com/flaresimulations/synthesizer/blob/76cee693fbd73ec44403aceb5648da773b95e57f/src/synthesizer/components/stellar.py#L467