preprocessor: do not destroy double slash escaped identifiers

The preprocessor currently destroys double slash containing escaped
identifiers (for example \a//b ). This is due to next_token trying to
convert single line comments (//) into /* */ comments. This then leads
to an unintuitive error message like this:
ERROR: syntax error, unexpected '*'

This patch fixes the error by recognizing escaped identifiers and
returning them as single token. It also adds a testcase.

tests/verilog/doubleslash.ys

@@ -0,0 +1,19 @@
+read_verilog -sv <<EOT
+module doubleslash
+  (input  logic   a,
+   input  logic   b,
+   output logic   z);
+
+  logic \a//b ;
+
+  assign \a//b = a & b;
+  assign z = ~\a//b ;
+
+endmodule : doubleslash
+EOT
+
+hierarchy
+proc
+opt -full
+
+write_verilog doubleslash.v