Add `native` Noise dialect to QASM2 output as a comment.

[weinbe58]

have a program like

@qasm2.extended
def kernel():
    q = qasm2.Greg(3)
    native.pauli_channel([q[0]], px=0.01, py=0.02, pz=0.03)
    qasm2.h(q[0])

would lower to the following text:

OPENQASM 2.0;
#include "qelibc1.inc"

qreg[3] q;
// pauli_channel(px=0.01, py=0.02, pz=0.03) { q[0] }
h(q[1]);

using a Comment AST node to represent the text

cc: @stefan-o @lamq317 @Roger-luo