Introduce avifDiagnostics, which allows for a detailed, freeform error string upon decode error

include/avif/internal.h

diff --git a/include/avif/internal.h b/include/avif/internal.h
index b068834..c6f23fe 100644
--- a/include/avif/internal.h
+++ b/include/avif/internal.h
@@ -265,6 +265,12 @@
 const char * avifCodecVersionSvt(void);   // requires AVIF_CODEC_SVT (codec_svt.c)
 
 // ---------------------------------------------------------------------------
+// avifDiagnostics
+
+void avifDiagnosticsClearError(avifDiagnostics * diag);
+void avifDiagnosticsPrintf(avifDiagnostics * diag, const char * format, ...);
+
+// ---------------------------------------------------------------------------
 // avifStream
 
 typedef size_t avifBoxMarker;