An ExGram.Router.Filter that matches when any FSM flow is active.
This filter is automatically registered as the :in_flow alias when
use ExGram.FSM detects that use ExGram.Router has also been called on
the same module.
Usage
Use in a scope to match when any flow is active:
scope do
filter :in_flow
handle &MyBot.Handlers.in_flow/1
end