traces: add some colors

This is an experiment.  Maybe more styles will be used (in which case
a short-hand function will be useful), maybe it will be just reverted.
* data/bison-default.css (.traces0): New.
* src/lalr.c (lalr): Use it.

data/bison-default.css

@@ -22,3 +22,4 @@
 .note      { color: cyan; }
 /* Semantic values in Bison's own parser traces.  */
 .value     { color: green; }
+.trace0    { color: green; }