modem: chat: Implement TRANSMIT_IDLE event

Refactors modem chat module to use TRANSMIT_IDLE event instead
of polling when transmitting, cleaning up the transmit handler
by combining duplicate code and using an internal state instead
of multiple iterators to track send state.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>
2 files changed