slang-netlist issue with net declaration assignments

[udif]

@jameshanlon
Describe the bug
It seems that slang-netlist ignores the assignment part of net declaration assignments.

To Reproduce
Please run the following file (test.v):

module t;
reg a, b;
wire c;
initial begin
  a <= 1;
  b <= a;
end
assign c = a;
wire d = a;
wire e = d;
endmodule

with

build/bin/slang-netlist --netlist-dot - test.v

The output we get is:

digraph {
  node [shape=record];
  N1 [label="Variable declaration\nt.a"]
  N2 [label="Variable declaration\nt.b"]
  N3 [label="Variable declaration\nt.c"]
  N4 [label="Variable declaration\nt.d"]
  N5 [label="Variable declaration\nt.e"]
  N6 [label="a[0:0]\n[Assigned to]"]
  N7 [label="b[0:0]\n[Assigned to]"]
  N8 [label="a[0:0]\n"]
  N9 [label="c[0:0]\n[Assigned to]"]
  N10 [label="a[0:0]\n"]
  N11 [label="Variable alias\nt.a"]
  N6 -> N11
  N7 -> N2
  N8 -> N7
  N9 -> N3
  N10 -> N9
  N11 -> N8
  N11 -> N10
}

As you can see, there is no reference to the d and e net declaration assignments. Their declaration nodes, N4 and N5 are not connected in any way to the rest of the netlist.

Additional context
I have a pending PR which prints a warning for unused variable declarations (pretty trivial but useful), which gets incorrectly triggered by those issues.