verilog backend: preserve signed on wire and port declarations

`dump_wire` had no code path that emits the `signed` keyword for
wires/ports whose RTLIL `is_signed` flag is set.  Reading

  module top(input signed [9:0] in, output signed [31:0] o);
    assign o = in;
  endmodule

and writing it back via `write_verilog` produced

  input  [9:0]  in;
  output [31:0] o;

losing the declared signedness even though `wire->is_signed` was
tracked correctly in RTLIL throughout the round trip.  The IEEE
1364-2001 grammar (Annex A.2.1.2 / A.2.1.3) allows `signed` after the
direction / net-type keyword, which is the dialect `write_verilog`
targets by default — so the fix is to emit ` signed` between the
direction/net-type and the range when `wire->is_signed`.

Closes the half of chipsalliance/synlig#2425 that lives in Yosys: the
SystemVerilog frontend correctly produces a signed wire for `output int`,
but the Verilog backend dropped it on write.

Adds `tests/various/write_verilog_signed_port.ys`, which round-trips a
module with `signed` inputs, outputs, and an internal wire and greps
for the keyword on each declaration — fails without the fix, passes
with it.

tests/various/write_verilog_signed_port.ys

@@ -0,0 +1,33 @@
+# Roundtrip a module with `signed` ports / wires through read_verilog
+# and write_verilog and verify the `signed` keyword survives.  Before
+# the fix, write_verilog silently dropped `signed` on every wire / port
+# declaration, even though wire->is_signed was tracked correctly
+# through the RTLIL.  The IEEE 1364-2001 grammar (Annex A.2.1.2 /
+# A.2.1.3) allows `signed` after the direction / net-type, which is
+# the dialect write_verilog targets by default.
+
+! mkdir -p temp
+read_verilog <<EOT
+module top(
+    input  signed [9:0]  in,
+    input  signed        s_in,
+    output signed [31:0] o,
+    output signed        s_o
+);
+    wire signed [15:0] w;
+    assign w   = in;
+    assign o   = w;
+    assign s_o = s_in;
+endmodule
+EOT
+
+write_verilog -noattr temp/write_verilog_signed_port.v
+
+# Each `signed` declaration must round-trip exactly.  Use a non-greedy
+# grep on the relevant slices (input / output / wire) so any drift
+# in the surrounding formatting doesn't mask the bug.
+! grep -E '^\s*input  *signed *\[9:0\] +in;'   temp/write_verilog_signed_port.v
+! grep -E '^\s*input  *signed +s_in;'          temp/write_verilog_signed_port.v
+! grep -E '^\s*output  *signed *\[31:0\] +o;'  temp/write_verilog_signed_port.v
+! grep -E '^\s*output  *signed +s_o;'          temp/write_verilog_signed_port.v
+! grep -E '^\s*wire  *signed *\[15:0\] +w;'    temp/write_verilog_signed_port.v