Better API for two qubit pauli channel

[johnzl-777]

@david-pl commented on this in #462 but I remember having a similar train of thought from #211. This isn't a blocker for anything but could make for better UX.

Right now you have to pass in a somewhat cumbersome and fragile list of floats to indicate the probability of each pauli pair being applied. The only way to know which float corresponds with which pair is to look at the documentation.

I like @david-pl 's idea of allowing a dictionary where each pauli pair representation (a string I imagine) goes to a float. So something like

params = {"IX": 0.1, "IY":0.2, ...}
squin.channel.two_qubit_pauli_channel(params, ...)

As an aside I originally was thinking of fixed, named arguments which is alright for single qubit pauli errors but I don't think would be great for 2 or beyond 😅