Introduce "command" styling

This adds a new "command" style that is used when styling the name of
a gdb command.

Note that not every instance of a command name that is output by gdb
is changed here.  There is currently no way to style error() strings,
and there is no way to mark up command help strings.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31747
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: Keith Seitz <keiths@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/exec.c

@@ -327,7 +327,8 @@ exec_file_locate_attach (int pid, int defer_bp_reset, int from_tty)
       warning (_("No executable has been specified and target does not "
 		 "support\n"
 		 "determining executable automatically.  "
-		 "Try using the \"file\" command."));
+		 "Try using the \"%ps\" command."),
+	       styled_string (command_style.style (), "file"));
       return;
     }