diagnostics: display suggested update after the caret-info

This commit adds the suggestion in green, on the line below the
caret-and-tildes.

    foo.y:1.1-14: warning: deprecated directive: '%error-verbose', use '%define parse.error verbose' [-Wdeprecated]
        1 | %error-verbose
          | ^~~~~~~~~~~~~~
          | %define parse.error verbose

The current approach, with location_caret_suggestion, is fragile:
there's a protocol of calls to the complain functions which is strict.
We should rather have a richer structure describing the diagnostics,
including with submessages such as the suggestions, passed in the end
to the routines in charge of formatting and printing them.

* src/location.h, src/location.c (location_caret_suggestion): New.
* src/complain.c (deprecated_directive): Use it.
* tests/diagnostics.at, tests/input.at: Adjust expectations.

src/location.c

@@ -470,6 +470,19 @@ location_caret (location loc, const char *style, FILE *out)
   }
 }
 
+void
+location_caret_suggestion (location loc, const char *s, FILE *out)
+{
+  const char *style = "insertion";
+  fprintf (out, "      | %*s",
+           loc.start.column - 1 - caret_info.skip + (caret_info.skip ? 3 : 0),
+           "");
+  begin_use_class (style, out);
+  fputs (s, out);
+  end_use_class (style, out);
+  putc ('\n', out);
+}
+
 bool
 location_empty (location loc)
 {