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.

data/bison-default.css

@@ -17,9 +17,14 @@
 /* This is an experimental feature.  The class names may change in the
    future.  */
 
+/* Diagnostics.  */
 .warning   { color: purple; }
 .error     { color: red; }
 .note      { color: cyan; }
+.insertion { color: green; }
+
 /* Semantic values in Bison's own parser traces.  */
 .value     { color: green; }
+
+/* "Sections" in traces (--trace).  */
 .trace0    { color: green; }