Add stdlib squin functions for power of pauli operators

[david-pl]

We can express powers of pauli operators with rotations.

Here's a snippet by @jon-wurtz for an xpowshorthand:

@squin.kernel
def xpow(angle:float):
    op = squin.op.x()
    return squin.op.rot(op,angle)

We can add a generic pow function that takes both an operator and a power creating a rotation with the given angle and forward calls for powers of other pauli operators to that.