diagnostics: clean up convention for colored diagnostics

* data/diagnostics.css: Rename as...
* data/bison-default.css: this.
Add the GPL header.
This is the convention followed by Bruno Haible in gettext.
Adjust dependencies.
* src/complain.c (complain_init_color): Use BISON_STYLE instead of
BISON_DIAGNOSTICS_STYLE.

data/bison-default.css

@@ -0,0 +1,24 @@
+/* Default styling rules for Bison when doing terminal output.
+   Copyright (C) 2006-2007 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* This is an experimental feature.  The class names may change in the
+   future.  */
+
+.warning   { color: purple; }
+.error     { color: red; }
+.note      { color: cyan; }
+/* Semantic values in Bison's own parser traces.  */
+.value     { color: green; }