Upgrade qml.commutator to use pauli arithmetic under the hood whenever possible

[Qottmann]

Upgrade qml.commutator to use pauli artihmetic whenever that is possible (i.e. whenever both operators have a pauli_rep. This was initiated in #5051 (comment) Similar things could be done in qml.dot and other points of the codebase. The grander idea is to use fater pauli arithmetic under the hood whenever that is possible.

The main feature of this update is performance:

op1 = qml.dot(range(30), [X(i) @ X(i+1) for i in range(30)])
op2 = qml.dot(range(30), [Y(i) @ Y(i+1) for i in range(30)])

%timeit qml.commutator(op1, op2) # on main branch
11.9 ms ± 32.2 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
%timeit qml.commutator(op1, op2) # this branch
1.28 ms ± 3.42 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

• Basic functionality
• Handle Zero case (empty PauliSentence) with 0 * Identity()
• Add test ensuring pauli route is taken
• All tests pass
• Test mixed cases (ops and Paulis)
• changelog

[sc-56865]

[albi3ro]

Just a request for PauliWord.pauli_rep to be a PauliSentence, since Operator.pauli_rep is always a PauliSentence. Might be worth a discussion about allowing Operator.pauli_rep to also be a PauliWord, but I think we can put off that discussion.