extensions FEATURE yang-data extension implementation

Internal libyang support for yang-data extension defined in
ietf-restconf module (RFC 8040)

Fixes #1334
diff --git a/src/plugins_exts_internal.h b/src/plugins_exts_internal.h
index fac88f4..ab6196c 100644
--- a/src/plugins_exts_internal.h
+++ b/src/plugins_exts_internal.h
@@ -31,6 +31,11 @@
 #define LYEXT_PLUGIN_INTERNAL_ANNOTATION 4
 
 /**
+ * @brief Index of yang-data extension plugin in lyext_plugins_internal
+ */
+#define LYEXT_PLUGIN_INTERNAL_YANGDATA 5
+
+/**
  * @brief Find the extension plugin for the specified extension instance.
  *
  * @param[in] mod YANG module where the